ci: add GitHub Actions workflow (lint, typecheck, test, build)

.github/workflows/ci.yml

@@ -0,0 +1,48 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [master]
+  workflow_dispatch:
+
+# Least privilege: the job only needs to read the repo.
+permissions:
+  contents: read
+
+# Cancel a PR's in-progress run when a new commit is pushed to the same ref.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  verify:
+    name: verify
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      # Ordered cheap → expensive for fast feedback. These mirror the local gates in CLAUDE.md.
+      - name: Lint
+        run: npm run lint
+
+      - name: Typecheck
+        run: npm run typecheck
+
+      - name: Test
+        run: npm run test
+
+      - name: Build
+        run: npm run build
+
+      # Fail loudly if the manifest copy step didn't produce a loadable extension.
+      - name: Verify build output
+        run: test -f dist/manifest.json