[CI] Add GitHub Actions workflow for lint + tests

[SaharPak]

Set up a GitHub Actions CI pipeline that runs on every PR.

The workflow should:

1. Install pnpm and Node.js 20
2. Run pnpm install
3. Run pnpm build (TypeScript compilation)
4. Run pnpm test (unit tests)
5. Run pnpm format:check (Prettier)

Create .github/workflows/ci.yml. Use pnpm caching for faster runs.

This is critical infrastructure. No other PRs should merge without CI passing.