Add GitHub Actions CI workflow

Runs lint, format check, build, and test on Node 20+22 for push/PR to main.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: meimakes

.github/workflows/ci.yml

@@ -0,0 +1,37 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [20, 22]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: npm
+
+      - run: npm ci
+
+      - name: Lint
+        run: npx eslint src/
+
+      - name: Format check
+        run: npx prettier --check 'src/**/*.ts'
+
+      - name: Build
+        run: npm run build
+
+      - name: Test
+        run: npm test