- Extract workspace-startup.ts with buildChatSummary, shouldRetryBootstrapWarmup,
  isTransientLocalGatewayError and toStartupErrorMessage for unified startup state
- Converge index.ts and ipc.ts to use runtimeManager.getGatewayConnection() as
  truth source for bundled-runtime mode, preventing fallback to persisted 18789
- Add WINDOWS_POWERSHELL_PATH / WINDOWS_TASKKILL_PATH absolute paths in
  runtime-manager to fix Git Bash path hijacking on Windows
- Buffer last 50 stderr lines and surface name-conflict / port-in-use hints in
  lastError via buildStderrHint()
- Add smoke:workspace-startup script and workspace-startup-smoke test
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>