Add GitHub Actions CI workflow

- Run tests on push to main and add-tests branches
- Test on Node.js 18.x, 20.x, and 22.x
- Run tests, build, and prettier formatting checks
- Use npm ci for faster, reliable installs
- Trigger on pull requests to main branch

Author: duncanmcclean

.github/workflows/ci.yml

@@ -0,0 +1,37 @@
+name: CI
+
+on:
+  push:
+    branches: [ main, add-tests ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [18.x, 20.x, 22.x]
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Setup 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 run test:run
+
+    - name: Run build
+      run: npm run build
+
+    - name: Check formatting
+      run: npm run prettier:check