[DO NOT MERGE] feat: Check Nix package is working as part of the CI workflow

.devcontainer/Dockerfile

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+FROM ubuntu:22.04
+
+ARG DEBIAN_FRONTEND=noninteractive
+# enable 'universe' because musl-tools & clang live there
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    software-properties-common && \
+    add-apt-repository --yes universe
+
+# now install build deps
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    build-essential curl git ca-certificates \
+    pkg-config clang musl-tools libssl-dev && \
+    rm -rf /var/lib/apt/lists/*
+
+# non-root dev user
+ARG USER=dev
+ARG UID=1000
+RUN useradd -m -u $UID $USER
+USER $USER
+
+# install Rust + musl target as dev user
+RUN curl -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal && \
+    ~/.cargo/bin/rustup target add aarch64-unknown-linux-musl
+
+ENV PATH="/home/${USER}/.cargo/bin:${PATH}"
+
+WORKDIR /workspace

.devcontainer/README.md

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+# Containerized Development
+
+We provide the following options to facilitate Codex development in a container. This is particularly useful for verifying the Linux build when working on a macOS host.
+
+## Docker
+
+To build the Docker image locally for x64 and then run it with the repo mounted under `/workspace`:
+
+```shell
+CODEX_DOCKER_IMAGE_NAME=codex-linux-dev
+docker build --platform=linux/amd64 -t "$CODEX_DOCKER_IMAGE_NAME" ./.devcontainer
+docker run --platform=linux/amd64 --rm -it -e CARGO_TARGET_DIR=/workspace/codex-rs/target-amd64 -v "$PWD":/workspace -w /workspace/codex-rs "$CODEX_DOCKER_IMAGE_NAME"
+```
+
+Note that `/workspace/target` will contain the binaries built for your host platform, so we include `-e CARGO_TARGET_DIR=/workspace/codex-rs/target-amd64` in the `docker run` command so that the binaries built inside your container are written to a separate directory.
+
+For arm64, specify `--platform=linux/amd64` instead for both `docker build` and `docker run`.
+
+Currently, the `Dockerfile` works for both x64 and arm64 Linux, though you need to run `rustup target add x86_64-unknown-linux-musl` yourself to install the musl toolchain for x64.
+
+## VS Code
+
+VS Code recognizes the `devcontainer.json` file and gives you the option to develop Codex in a container. Currently, `devcontainer.json` builds and runs the `arm64` flavor of the container.
+
+From the integrated terminal in VS Code, you can build either flavor of the `arm64` build (GNU or musl):
+
+```shell
+cargo build --target aarch64-unknown-linux-musl
+cargo build --target aarch64-unknown-linux-gnu
+```

.devcontainer/devcontainer.json

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+{
+  "name": "Codex",
+  "build": {
+    "dockerfile": "Dockerfile",
+    "context": "..",
+    "platform": "linux/arm64"
+  },
+
+  /* Force VS Code to run the container as arm64 in
+     case your host is x86 (or vice-versa). */
+  "runArgs": ["--platform=linux/arm64"],
+
+  "containerEnv": {
+    "RUST_BACKTRACE": "1",
+    "CARGO_TARGET_DIR": "${containerWorkspaceFolder}/codex-rs/target-arm64"
+  },
+
+  "remoteUser": "dev",
+  "customizations": {
+    "vscode": {
+      "settings": {
+          "terminal.integrated.defaultProfile.linux": "bash"
+      },
+      "extensions": [
+          "rust-lang.rust-analyzer"
+      ],
+    }
+  }
+}

.github/actions/codex/.gitignore

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+/node_modules/

.github/actions/codex/.prettierrc.toml

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+printWidth = 80
+quoteProps = "consistent"
+semi = true
+tabWidth = 2
+trailingComma = "all"
+
+# Preserve existing behavior for markdown/text wrapping.
+proseWrap = "preserve"

.github/actions/codex/README.md

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+# openai/codex-action
+
+`openai/codex-action` is a GitHub Action that facilitates the use of [Codex](https://github.com/openai/codex) on GitHub issues and pull requests. Using the action, associate **labels** to run Codex with the appropriate prompt for the given context. Codex will respond by posting comments or creating PRs, whichever you specify!
+
+Here is a sample workflow that uses `openai/codex-action`:
+
+```yaml
+name: Codex
+
+on:
+  issues:
+    types: [opened, labeled]
+  pull_request:
+    branches: [main]
+    types: [labeled]
+
+jobs:
+  codex:
+    if: ... # optional, but can be effective in conserving CI resources
+    runs-on: ubuntu-latest
+    # TODO(mbolin): Need to verify if/when `write` is necessary.
+    permissions:
+      contents: write
+      issues: write
+      pull-requests: write
+    steps:
+      # By default, Codex runs network disabled using --full-auto, so perform
+      # any setup that requires network (such as installing dependencies)
+      # before openai/codex-action.
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Run Codex
+        uses: openai/codex-action@latest
+        with:
+          openai_api_key: ${{ secrets.CODEX_OPENAI_API_KEY }}
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+```
+
+See sample usage in [`codex.yml`](../../workflows/codex.yml).
+
+## Triggering the Action
+
+Using the sample workflow above, we have:
+
+```yaml
+on:
+  issues:
+    types: [opened, labeled]
+  pull_request:
+    branches: [main]
+    types: [labeled]
+```
+
+which means our workflow will be triggered when any of the following events occur:
+
+- a label is added to an issue
+- a label is added to a pull request against the `main` branch
+
+### Label-Based Triggers
+
+To define a GitHub label that should trigger Codex, create a file named `.github/codex/labels/LABEL-NAME.md` in your repository where `LABEL-NAME` is the name of the label. The content of the file is the prompt template to use when the label is added (see more on [Prompt Template Variables](#prompt-template-variables) below).
+
+For example, if the file `.github/codex/labels/codex-review.md` exists, then:
+
+- Adding the `codex-review` label will trigger the workflow containing the `openai/codex-action` GitHub Action.
+- When `openai/codex-action` starts, it will replace the `codex-review` label with `codex-review-in-progress`.
+- When `openai/codex-action` is finished, it will replace the `codex-review-in-progress` label with `codex-review-completed`.
+
+If Codex sees that either `codex-review-in-progress` or `codex-review-completed` is already present, it will not perform the action.
+
+As determined by the [default config](./src/default-label-config.ts), Codex will act on the following labels by default:
+
+- Adding the `codex-review` label to a pull request will have Codex review the PR and add it to the PR as a comment.
+- Adding the `codex-triage` label to an issue will have Codex investigate the issue and report its findings as a comment.
+- Adding the `codex-issue-fix` label to an issue will have Codex attempt to fix the issue and create a PR wit the fix, if any.
+
+## Action Inputs
+
+The `openai/codex-action` GitHub Action takes the following inputs
+
+### `openai_api_key` (required)
+
+Set your `OPENAI_API_KEY` as a [repository secret](https://docs.github.com/en/actions/security-for-github-actions/security-guides/using-secrets-in-github-actions). See **Secrets and varaibles** then **Actions** in the settings for your GitHub repo.
+
+Note that the secret name does not have to be `OPENAI_API_KEY`. For example, you might want to name it `CODEX_OPENAI_API_KEY` and then configure it on `openai/codex-action` as follows:
+
+```yaml
+openai_api_key: ${{ secrets.CODEX_OPENAI_API_KEY }}
+```
+
+### `github_token` (required)
+
+This is required so that Codex can post a comment or create a PR. Set this value on the action as follows:
+
+```yaml
+github_token: ${{ secrets.GITHUB_TOKEN }}
+```
+
+### `codex_args`
+
+A whitespace-delimited list of arguments to pass to Codex. Defaults to `--full-auto`, but if you want to override the default model to use `o3`:
+
+```yaml
+codex_args: "--full-auto --model o3"
+```
+
+For more complex configurations, use the `codex_home` input.
+
+### `codex_home`
+
+If set, the value to use for the `$CODEX_HOME` environment variable when running Codex. As explained [in the docs](https://github.com/openai/codex/tree/main/codex-rs#readme), this folder can contain the `config.toml` to configure Codex, custom instructions, and log files.
+
+This should be a relative path within your repo.
+
+## Prompt Template Variables
+
+As shown above, `"prompt"` and `"promptPath"` are used to define prompt templates that will be populated and passed to Codex in response to certain events. All template variables are of the form `{CODEX_ACTION_...}` and the supported values are defined below.
+
+### `CODEX_ACTION_ISSUE_TITLE`
+
+If the action was triggered on a GitHub issue, this is the issue title.
+
+Specifically it is read as the `.issue.title` from the `$GITHUB_EVENT_PATH`.
+
+### `CODEX_ACTION_ISSUE_BODY`
+
+If the action was triggered on a GitHub issue, this is the issue body.
+
+Specifically it is read as the `.issue.body` from the `$GITHUB_EVENT_PATH`.
+
+### `CODEX_ACTION_GITHUB_EVENT_PATH`
+
+The value of the `$GITHUB_EVENT_PATH` environment variable, which is the path to the file that contains the JSON payload for the event that triggered the workflow. Codex can use `jq` to read only the fields of interest from this file.
+
+### `CODEX_ACTION_PR_DIFF`
+
+If the action was triggered on a pull request, this is the diff between the base and head commits of the PR. It is the output from `git diff`.
+
+Note that the content of the diff could be quite large, so is generally safer to point Codex at `CODEX_ACTION_GITHUB_EVENT_PATH` and let it decide how it wants to explore the change.

.github/actions/codex/action.yml

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+name: "Codex [reusable action]"
+description: "A reusable action that runs a Codex model."
+
+inputs:
+  openai_api_key:
+    description: "The value to use as the OPENAI_API_KEY environment variable when running Codex."
+    required: true
+  trigger_phrase:
+    description: "Text to trigger Codex from a PR/issue body or comment."
+    required: false
+    default: ""
+  github_token:
+    description: "Token so Codex can comment on the PR or issue."
+    required: true
+  codex_args:
+    description: "A whitespace-delimited list of arguments to pass to Codex. Due to limitations in YAML, arguments with spaces are not supported. For more complex configurations, use the `codex_home` input."
+    required: false
+    default: "--config hide_agent_reasoning=true --full-auto"
+  codex_home:
+    description: "Value to use as the CODEX_HOME environment variable when running Codex."
+    required: false
+  codex_release_tag:
+    description: "The release tag of the Codex model to run."
+    required: false
+    default: "codex-rs-ca8e97fcbcb991e542b8689f2d4eab9d30c399d6-1-rust-v0.0.2505302325"
+
+runs:
+  using: "composite"
+  steps:
+    # Do this in Bash so we do not even bother to install Bun if the sender does
+    # not have write access to the repo.
+    - name: Verify user has write access to the repo.
+      env:
+        GH_TOKEN: ${{ github.token }}
+      shell: bash
+      run: |
+        set -euo pipefail
+
+        PERMISSION=$(gh api \
+          "/repos/${GITHUB_REPOSITORY}/collaborators/${{ github.event.sender.login }}/permission" \
+          | jq -r '.permission')
+
+        if [[ "$PERMISSION" != "admin" && "$PERMISSION" != "write" ]]; then
+          exit 1
+        fi
+
+    - name: Download Codex
+      env:
+        GH_TOKEN: ${{ github.token }}
+      shell: bash
+      run: |
+        set -euo pipefail
+
+        # Determine OS/arch and corresponding Codex artifact name.
+        uname_s=$(uname -s)
+        uname_m=$(uname -m)
+
+        case "$uname_s" in
+          Linux*)   os="linux" ;;
+          Darwin*)  os="apple-darwin" ;;
+          *) echo "Unsupported operating system: $uname_s"; exit 1 ;;
+        esac
+
+        case "$uname_m" in
+          x86_64*) arch="x86_64" ;;
+          arm64*|aarch64*) arch="aarch64" ;;
+          *) echo "Unsupported architecture: $uname_m"; exit 1 ;;
+        esac
+
+        # linux builds differentiate between musl and gnu.
+        if [[ "$os" == "linux" ]]; then
+          if [[ "$arch" == "x86_64" ]]; then
+            triple="${arch}-unknown-linux-musl"
+          else
+            # Only other supported linux build is aarch64 gnu.
+            triple="${arch}-unknown-linux-gnu"
+          fi
+        else
+          # macOS
+          triple="${arch}-apple-darwin"
+        fi
+
+        # Note that if we start baking version numbers into the artifact name,
+        # we will need to update this action.yml file to match.
+        artifact="codex-exec-${triple}.tar.gz"
+
+        gh release download ${{ inputs.codex_release_tag }} --repo openai/codex \
+          --pattern "$artifact" --output - \
+        | tar xzO > /usr/local/bin/codex-exec
+        chmod +x /usr/local/bin/codex-exec
+
+        # Display Codex version to confirm binary integrity; ensure we point it
+        # at the checked-out repository via --cd so that any subsequent commands
+        # use the correct working directory.
+        codex-exec --cd "$GITHUB_WORKSPACE" --version
+
+    - name: Install Bun
+      uses: oven-sh/setup-bun@v2
+      with:
+        bun-version: 1.2.11
+
+    - name: Install dependencies
+      shell: bash
+      run: |
+        cd ${{ github.action_path }}
+        bun install --production
+
+    - name: Run Codex
+      shell: bash
+      run: bun run ${{ github.action_path }}/src/main.ts
+      # Process args plus environment variables often have a max of 128 KiB,
+      # so we should fit within that limit?
+      env:
+        INPUT_CODEX_ARGS: ${{ inputs.codex_args || '' }}
+        INPUT_CODEX_HOME: ${{ inputs.codex_home || ''}}
+        INPUT_TRIGGER_PHRASE: ${{ inputs.trigger_phrase || '' }}
+        OPENAI_API_KEY: ${{ inputs.openai_api_key }}
+        GITHUB_TOKEN: ${{ inputs.github_token }}
+        GITHUB_EVENT_ACTION: ${{ github.event.action || '' }}
+        GITHUB_EVENT_LABEL_NAME: ${{ github.event.label.name || '' }}
+        GITHUB_EVENT_ISSUE_NUMBER: ${{ github.event.issue.number || '' }}
+        GITHUB_EVENT_ISSUE_BODY: ${{ github.event.issue.body || '' }}
+        GITHUB_EVENT_REVIEW_BODY: ${{ github.event.review.body || '' }}
+        GITHUB_EVENT_COMMENT_BODY: ${{ github.event.comment.body || '' }}