ci: add GitHub Actions PR workflow (compatibility proof)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/pr.yml

@@ -0,0 +1,23 @@
+# GitHub-compatibility proof (Plan 10): the same npm scripts the Forgejo
+# composite actions run, using only stock GitHub actions. Forgejo ignores
+# .github/ because .forgejo/workflows/ exists.
+name: pr
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+jobs:
+  checks:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version-file: .nvmrc
+          cache: npm
+      - run: npm ci
+      - run: npm run lint
+      - run: npm run format:check
+      - run: npm test
+      - run: npm run build