ci/stitchmd: Don't build from source, use action (uber-go#180)

This replaces the manual compilation and running of stitchmd
with the [stitchmd-action](https://github.com/abhinav/stitchmd-action).
The action downloads a pre-built copy of stitchmd from GitHub,
and runs it.

The action offers multiple execution modes. Those relevant here are:

- check: fail if the file is not up-to-date
- write: update the file if it's not up-to-date

The workflow added here uses both:

- For a push to any branch, it will run in check mode. Read-only.
  It'll check that the file is up-to-date and that's it.
- However, if a pull request is created with a branch,
  it will run in write mode -- automatically updating the PR if needed.

This has some nice properties in terms of UX and safety:

- We never push to a branch without permission.
  This alleviates safety concerns around automated pushes.
- We make it easy to make edits on the GitHub UI,
  and still have them be picked up by the system
  without creating a local checkout
- Setting up Go, building stitchmd from source, and then running it--all
  this takes some time.
  It's *a lot* faster to download and run the binary.

The instructions taken to set up this dual mode
can be found here:
https://github.com/abhinav/stitchmd-action#automatically-update-for-prs-only

Note that this nukes the tools/ directory, but keeps the `Makefile`
so that if someone wants to run this locally, they still can.

.github/workflows/ci.yml

@@ -7,19 +7,34 @@ on:
     branches: [ '*' ]
 
 jobs:
-
-  build:
+  stitchmd:
+    name: Check or update style.md
     runs-on: ubuntu-latest
 
+    # Needed to give the job permission
+    # to push to branches.
+    permissions:
+      contents: write
+
     steps:
-    - uses: actions/checkout@v3
+    - name: Check out repository
+      uses: actions/checkout@v3
 
-    - name: Set up Go
-      uses: actions/setup-go@v4
+    - name: Check or update style.md
+      uses: abhinav/stitchmd-action@v1
       with:
-        go-version: 1.20.x
-        cache: true
-        cache-dependency-path: tools/go.sum
+        # For pull requests, run in 'write' mode so that edits made
+        # directly in the GitHub UI get propagated to style.md
+        # without a local checkout.
+        #
+        # Otherwise, run in 'check' mode to fail CI if style.md is out-of-date.
+        mode: ${{ github.event_name == 'pull_request' && 'write' || 'check' }}
+        summary: src/SUMMARY.md
+        preface: src/preface.txt
+        output: style.md
 
-    - name: Lint
-      run: make lint
+    - uses: stefanzweifel/git-auto-commit-action@v4
+      if: ${{ github.event_name == 'pull_request' }}
+      with:
+        file_pattern: style.md
+        commit_message: 'Auto-update style.md'

Makefile

@@ -4,6 +4,8 @@ SHELL = /bin/bash
 export GOBIN ?= $(dir $(abspath $(lastword $(MAKEFILE_LIST))))/bin
 
 STITCHMD = $(GOBIN)/stitchmd
+
+# Keep these options in-sync with .github/workflows/ci.yml.
 STITCHMD_ARGS = -o style.md -preface src/preface.txt src/SUMMARY.md
 
 .PHONY: all
@@ -21,5 +23,5 @@ lint: $(STITCHMD)
 style.md: $(STITCHMD) $(wildcard src/*)
 	$(STITCHMD) $(STITCHMD_ARGS)
 
-$(STITCHMD): tools/go.mod
-	go install -C tools go.abhg.dev/stitchmd
+$(STITCHMD):
+	go install go.abhg.dev/stitchmd@latest