Add CI workflow to run tests on pull requests

- Run tests on PRs to main and pushes to main
- Uses Node.js 20 with npm caching for faster runs
- Builds project and runs vitest test suite

Agent-Id: agent-197d346e-17e3-4016-8f9a-c7d77e060a1e

Author: richhankins

.github/workflows/ci.yml

@@ -0,0 +1,30 @@
+name: CI
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build
+        run: npm run build
+
+      - name: Run tests
+        run: npm test