ci: add GitHub Actions workflow for automated tests

Runs npm test on push and pull requests to main.
Matrix covers Node.js 22.x and 24.x.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    name: Tests (Node.js ${{ matrix.node-version }})
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [22.x, 24.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 ci
+
+      - name: Run tests
+        run: npm test