fix: restore --version and staged adapter shutdown fallback

[dutifulbob]

Summary

Restore two regressions introduced during the #28 refactor:

• bring back CLI --version output
• restore staged adapter shutdown fallback in AcpClient.close()
  • close stdin first
  • wait briefly
  • send SIGTERM and wait
  • force SIGKILL if still running

Details

--version

• reconnected commander version wiring with getAcpxVersion() in src/cli-core.ts
• restored regression test asserting acpx --version prints package.json version

Graceful shutdown fallback

• restored shutdown helpers in src/client.ts:
  • process-running detection
  • bounded wait-for-exit
  • staged signal escalation
• kept explicit stdio handle destruction to avoid hung CLI exits
• only unref() when process still has not exited after escalation
• reintroduced mock-agent --ignore-sigterm mode and added CLI regression test verifying sessions ensure exits cleanly in that case

Validation

• npm run lint
• npm run typecheck
• npm test