ci: run tests on push and pull request

.github/workflows/ci.yml

@@ -0,0 +1,22 @@
+# Run the test suite on every push and pull request. Tests are offline by
+# design (fixtures in tests/pages), so CI never flakes on a live site.
+name: ci
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 24
+      - run: npm ci
+      - run: npm test