Add hints after `/compact` command completes

Currently, codex cli does not give any indication when the /compact command is completed, which is confusing: users don’t know whether we have successfully performed the compaction:
<img width="2300" height="598" alt="Image" src="https://github.com/user-attachments/assets/33667ab5-8ed9-4878-b936-1571b59335d3" />

If codex client give a hint after compaction complete, it would be more helpful:
<img width="286" height="276" alt="Image" src="https://github.com/user-attachments/assets/62f3cf77-7988-49c6-9d78-5306df1f9eb5" />