Add GitHub Actions CI workflow

- Runs typecheck and tests on push/PR to master
- Publishes to npm on master push (requires NPM_TOKEN secret)

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

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: "npm"
+      - run: npm ci
+      - run: npm run typecheck
+      - run: npm test
+
+  publish:
+    needs: test
+    runs-on: ubuntu-latest
+    if: github.event_name == 'push' && github.ref == 'refs/heads/master'
+    permissions:
+      id-token: write
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: "npm"
+      - run: npm ci
+      - run: npm run build
+      - run: npm publish --access public
+        env:
+          NPM_TOKEN: ${{ secrets.NPM_TOKEN }}