Add a GitHub Actions–based CI pipeline

Question

**Why:** Catches regressions on every push/PR, enforces lint, type‑check, tests and build.

**Scope & plan**

Create `.github/workflows/ci.yml` that on `push` and `pull_request`:
                 • `npm ci` in `codex-cli/`
                 • `npm run lint`
                 • `npm run typecheck`
                 • `npm test`
                 • `npm run build`

Happy to get feedback and send PR.