Add GitHub Actions CI (lint + test + build)

[BondarenkoCom]

Summary

The repo has no CI. A basic GitHub Actions workflow would catch regressions early.

What to implement

• .github/workflows/ci.yml
• Steps: checkout, Node 20 setup, npm ci, npm run check (TypeScript), npm test
• Trigger on push to main + PRs
• Optional: npm run build to verify full compilation

Good first issue?

Yes — standard GitHub Actions setup. No custom tooling needed.