ci: add GitHub Actions workflow for lint, typecheck, and test

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI — Lint, Typecheck, Test
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: web
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+          cache-dependency-path: web/package-lock.json
+
+      - run: npm ci
+
+      - name: Typecheck
+        run: npx tsc --noEmit
+
+      - name: Lint
+        run: npm run lint
+
+      - name: Test
+        run: npm test