feat: add CI workflow to typecheck and build on PRs

[GreyC]

Problem

There is no CI check on pull requests. Type errors and build failures only surface after merge (or during code review). The @types/inquirer mismatch in PR #6 would have been caught automatically.

Fix

Add .github/workflows/ci.yml that runs on every PR:
```yaml

• Install deps: npm install
• Build: npm run build (tsc)
  ```
  Use self-hosted runner to match the publish workflow.

Acceptance Criteria

• PRs targeting main trigger a build check
• TypeScript errors fail the check