Add plugins matrix job to CI for tsc, lint, prettier, and build (#15)

* Add plugins matrix job to CI for tsc, lint, prettier, and build

Runs yarn install --immutable, type checking, linting, formatting check,
and build for each plugin workspace in parallel.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Christoph Jerolimov <jerolimov+git@redhat.com>

* Disable fail-fast, add plugins-check job, and disable prettier step

Allow all matrix jobs to complete independently and add a gate job
to verify all passed. Temporarily disable prettier:check.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Christoph Jerolimov <jerolimov+git@redhat.com>

* Add .github/ drift check to plugins matrix job

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Christoph Jerolimov <jerolimov+git@redhat.com>

* Split plugins matrix into a separate CI workflow

Move the plugins matrix and plugins-check jobs out of ci.yaml into a
dedicated ci-plugins.yaml workflow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Christoph Jerolimov <jerolimov+git@redhat.com>

---------

Signed-off-by: Christoph Jerolimov <jerolimov+git@redhat.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: christoph-jerolimov

.github/workflows/ci-plugins.yaml

@@ -0,0 +1,84 @@
+name: CI plugins
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  plugins:
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        workspace:
+          - backstage-1.42
+          - backstage-1.45
+          # - backstage-1.49
+          # - backstage-1.52
+
+    steps:
+      - name: Harden runner
+        uses: step-security/harden-runner@v2
+        with:
+          egress-policy: audit
+
+      - name: Checkout
+        uses: actions/checkout@v7
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version-file: plugins/${{ matrix.workspace }}/package.json
+
+      - name: Install dependencies
+        run: yarn install --immutable
+        working-directory: plugins/${{ matrix.workspace }}
+
+      - name: TypeScript type check
+        run: yarn tsc
+        working-directory: plugins/${{ matrix.workspace }}
+
+      - name: Lint
+        run: yarn lint:all
+        working-directory: plugins/${{ matrix.workspace }}
+
+      # - name: Check formatting
+      #   run: yarn prettier:check
+      #   working-directory: plugins/${{ matrix.workspace }}
+
+      - name: Build
+        run: yarn build:all
+        working-directory: plugins/${{ matrix.workspace }}
+
+      - name: Verify no changes in .github/
+        run: |
+          if [ -n "$(git status --porcelain .github/)" ]; then
+            echo "::error::Unexpected changes detected in .github/ after CI steps:"
+            git diff .github/
+            exit 1
+          fi
+          echo "No changes in .github/ — OK"
+
+  plugins-check:
+    if: always()
+    needs: plugins
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Harden runner
+        uses: step-security/harden-runner@v2
+        with:
+          egress-policy: audit
+
+      - name: Verify all plugin matrix jobs passed
+        run: |
+          if [ "${{ needs.plugins.result }}" != "success" ]; then
+            echo "::error::One or more plugin matrix jobs failed"
+            exit 1
+          fi