ci: test and build the beacon on every pull request

.github/workflows/ci.yml

@@ -0,0 +1,28 @@
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: read
+
+name: ci
+
+jobs:
+  beacon:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - uses: actions/setup-node@949feb2413d6458794dcd2491c4babbbce0c15c1 # v7.1.0
+        with:
+          node-version: 22
+      - run: npm ci --ignore-scripts
+      - run: npm test
+      - run: npm run build
+      # beacon.js is committed, so it has to match what its source builds,
+      # and the build must not write a file nobody committed
+      - run: git diff --exit-code
+      - run: git status --porcelain && test -z "$(git status --porcelain)"