feat: AI session title generation and /rename command

[EugeneTheDev]

Summary

• Adds TitleGenerator: after the first turn of a new session, generates a 3–7 word title via a short ephemeral Codex thread and persists it with thread/name/set. Generation is skipped if the session already has an explicit title or was resumed with an unknown history.
• Adds /rename as a built-in command in CodexCommands, which calls thread/name/set directly. The resulting thread/name/updated notification from Codex propagates to the ACP client as a session_info_update automatically through the existing notification handler.
• Adds threadSetName to CodexAppServerClient and renameSession to CodexAcpClient to support both features.

Notes

• Title generation is fire-and-forget and never blocks the prompt response.
• If the user renames the session (via /rename or any other means) after AI title generation has run, that explicit rename still propagates to the client via the existing thread/name/updated → session_info_update path.