fix: address review feedback and add preflight tests
- Consistent error formatting: single spawn now uses err.message like
batch-spawn, avoiding redundant "Error:" prefix
- checkBuilt distinguishes missing node_modules ("pnpm install") from
missing dist ("pnpm build") so users get the right guidance
- Add 13 preflight unit tests covering port, build, tmux, and gh checks
- Add 5 spawn integration tests covering runtime-aware tmux check,
tracker-aware gh check, and error message formatting
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>