Feature request: built-in notifications for CLI responses

### Summary

Provide a built-in way for Codex CLI to trigger desktop notifications (or expose an event stream) whenever the assistant finishes a response.

Motivation

I am using Codex CLI 0.42.0 on Windows and would like a toast notification each time the assistant replies. I tried implementing this by tailing .codex/sessions/*.jsonl and watching for agent_message events, but the current approach is fragile:

1. The CLI writes large base64 payloads into the session logs, which causes Windows Defender to quarantine the active .jsonl file as TrojanDownloader:VBS/Powdown.D!MSR.
2. Once Defender takes action, any watcher loop starts throwing Access is denied errors and stops delivering notifications.
3. Because the session logs are only intended for archival, there is no supported hook or API to stream responses in real time without touching those files.

Desired solution

Add an official notification feature (for example a --notify flag that fires a toast on Windows/macOS/Linux) or expose a stable event stream/pipe that external tooling can consume without having to touch the session archives.

Additional context

• Codex CLI 0.42.0
• Windows Security quarantines the session JSON; repeated accesses then fail with Test-Path : Access is denied.
• A builtin option would avoid security false positives and make the feature reliable for everyone.