feat: add GitHub Actions CI pipeline

[elicoon]

Summary

• Adds .github/workflows/ci.yml triggered on push to master and all PRs
• Runs: checkout → Node 20 setup (with npm cache) → npm ci → npm run build → npm run test
• Uses actions/checkout@v4 and actions/setup-node@v4

Test plan

• CI run triggered automatically after push
• All steps pass (npm ci, build, test)
• Green check visible on PR