Add coverage CI job

.github/workflows/coverage.yml

@@ -0,0 +1,76 @@
+name: Coverage
+
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    env:
+      TURBO_TOKEN: ${{ secrets.TURBO_TOKEN }}
+      TURBO_TEAM: ${{ secrets.TURBO_TEAM }}
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # Required for turbo diff
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v3
+        with:
+          version: 10
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: "pnpm"
+          cache-dependency-path: "**/pnpm-lock.yaml"
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+        # --- PR coverage (affected only) ---
+      - name: Run PR coverage
+        # the filter means 'projects changed since origin/main, inc dependents'
+        run: pnpm turbo coverage --filter=...[origin/main] --force --no-daemon
+
+      - name: Save PR coverage
+        run: |
+          mkdir pr-coverage
+          find . -name coverage-summary.json | while read file; do
+            project=$(dirname "$(dirname "$file")")
+            project=$(basename "$project")
+            cp "$file" "pr-coverage/$project.json"
+          done
+
+      - name: Capture PR ref
+        run: echo "PR_REF=${{ github.head_ref }}" >> $GITHUB_ENV
+
+        # --- baseline coverage ---
+      - name: Checkout main
+        run: git checkout origin/main
+
+      - name: Install dependencies (main)
+        run: pnpm install --frozen-lockfile
+
+      - name: Run coverage (main affected)
+        # notice both times we --force turbo to ignore cache and execute tasks fresh
+        run: pnpm turbo coverage --filter=...[origin/$PR_REF] --force --no-daemon
+
+      - name: Save coverage (main affected)
+        run: |
+          mkdir base-coverage
+          find . -name coverage-summary.json | while read file; do
+            project=$(dirname "$(dirname "$file")")
+            project=$(basename "$project")
+            cp "$file" "base-coverage/$project.json"
+          done
+
+        # --- compare ---
+      - name: Compare coverage
+        # the script errors (exit code 1), causing this action to fail,
+        # if regression for any app or package is detected
+        run: node scripts/compare-coverage.mjs pr-coverage base-coverage