ci: add GitHub Actions workflow for automated testing

Added CI workflow that runs on push and pull requests:
- Tests on Node.js 18.x, 20.x, and 22.x
- Runs typecheck, lint, build, and tests
- Separate job for test coverage reporting
- Uses npm ci for faster, reproducible builds
- Caches node_modules for improved performance

Author: katspaugh

.github/workflows/ci.yml

@@ -0,0 +1,61 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    name: Test on Node.js ${{ matrix.node-version }}
+    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: Type check
+        run: npm run typecheck
+
+      - name: Lint
+        run: npm run lint
+
+      - name: Build
+        run: npm run build
+
+      - name: Run tests
+        run: npm test -- --run
+
+  coverage:
+    name: Test Coverage
+    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.x
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run tests with coverage
+        run: npm test -- --run --coverage