ci: skip the build jobs for pull requests that only change Markdown or docs/ (#34)

docs/features.md still counts as code (a test checks it against the catalogue). Pushes to main
and manual runs always run every job.


Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -17,8 +17,45 @@ env:
   CARGO_TERM_COLOR: always
 
 jobs:
+  # Pull requests that only touch Markdown or docs/ skip the build jobs (they still report, as
+  # skipped). Pushes to main and manual runs always run everything.
+  changes:
+    name: Select checks
+    runs-on: ubuntu-24.04
+    outputs:
+      code: ${{ steps.paths.outputs.code }}
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          fetch-depth: 0
+      - id: paths
+        name: Detect code changes
+        env:
+          EVENT_NAME: ${{ github.event_name }}
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+        run: |
+          if [ "$EVENT_NAME" != "pull_request" ]; then
+            echo "code=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          changed=$(git diff --name-only "$BASE_SHA" "$HEAD_SHA")
+          code=false
+          while IFS= read -r f; do
+            case "$f" in
+              docs/features.md) code=true ;;
+              *.md|docs/*) ;;
+              "") ;;
+              *) code=true ;;
+            esac
+          done <<< "$changed"
+          echo "code=$code" >> "$GITHUB_OUTPUT"
+          echo "code changes: $code"
+
   ci:
     name: cargo xtask ci
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-24.04
     timeout-minutes: 60
     steps:
@@ -32,6 +69,8 @@ jobs:
 
   advisories:
     name: Dependency advisories and sources
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-24.04
     timeout-minutes: 20
     steps: