infra: add GitHub Actions CI (build + test on Node 22.x)

Runs on push and PR to main. Executes npm install, npm run build,
and npm test (131 tests) on ubuntu-latest with Node 22.x.

Closes #2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: TheStack-ai

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    name: build + test (Node ${{ matrix.node-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        node-version: ['22.x']
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm install
+
+      - name: Build
+        run: npm run build
+
+      - name: Run tests
+        run: npm test