Add Linux CI workflow with NuGet caching

[Copilot]

Changes

• .github/workflows/ci.yml: Ubuntu-latest workflow with restore/build/test pipeline

  • Triggers: push to main or master, all PRs
  • .NET 10.0.x via actions/setup-dotnet@v4
  • NuGet package caching on ~/.nuget/packages (keyed by csproj hashes)
  • Release configuration builds

• README.md: CI status badge added below title

Workflow Steps

- Setup .NET 10.0.x
- Cache ~/.nuget/packages
- dotnet restore
- dotnet build --configuration Release --no-restore  
- dotnet test --configuration Release --no-build

Foundation for platform-specific CI, packaging, and automated validation on every push.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>CI: Linux build + test workflow</issue_title>
<issue_description>## Problem

No .github/workflows/ directory exists. For a library targeting multiple platforms with unsafe code and P/Invoke bindings, automated CI is essential. This is the single highest-leverage task - everything else benefits from a green/red signal on every push.

Sub-tasks

• Create .github/workflows/ci.yml
• Jobs: dotnet restore, dotnet build --configuration Release, dotnet test
• Trigger on push to master and all PRs
• Ubuntu-latest runner
• Cache NuGet packages (actions/cache on ~/.nuget/packages)
• Add build status badge to README

Dependencies

Depends on: nothing
Unlocks: nearly everything - test sub-issues, benchmarks, platform CI, packaging

Phase

Phase 0 (Foundation) - can start immediately</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes CI: Linux build + test workflow #99

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

Pull request overview

Adds a basic Linux CI pipeline to validate Magma builds/tests on every change, helping catch cross-platform regressions early.

Changes:

• Introduces a GitHub Actions workflow to restore/build/test on ubuntu-latest with Release configuration.
• Adds NuGet package caching to speed up CI runs.
• Adds a CI status badge to README.md.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
.github/workflows/ci.yml	New Ubuntu CI workflow with NuGet caching and restore/build/test steps.
README.md	Adds a CI badge linking to the new workflow.

[Copilot]

The workflow is configured to trigger only for the master branch, but this PR appears to target main (and many repos use main as the default branch). As written, CI may never run on pushes/PRs to the default branch. Update the on.push.branches / on.pull_request.branches filters to match the repo’s default branch (or include both main and master).

Suggested change

	branches: [master]
	pull_request:
	branches: [master]
	branches: [main, master]
	pull_request:
	branches: [main, master]

[Copilot]

Updated in commit 078edaf to trigger on both main and master branches for future-proofing.

[benaadams]

@copilot apply changes based on the comments in this thread

[benaadams]

@copilot rebase to master and apply changes based on the comments in #146 (review)