Add CI: eslint + tsc + vitest

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DVibvSyvPsUXRXRYaFsWjY

Author: ampledata

.github/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+    tags: ['v*']
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npx eslint src/
+      - run: npx tsc --noEmit
+      - run: npx vitest run