Codex App: Add subagent monitoring to the UI

What variant of Codex are you using?

App

What feature would you like to see?

Right now, the app does not provide any indication that a subagent has been spawned and is being waited on. ATM, users are only shown a "Thinking" indicator when the agent is spawning subagents and waiting on them. It leads to the agent operating over extended periods without providing feedback to the user.

Minimal implementation request: The thread should display the same subagent events as the CLI.
• Agent spawned
• Waiting for agents
• Wait complete
• Agent closed

Nice to have: A subagent pane that displays the status of each spawned agent. Users should be able to monitor each subagent's progress (commands run, edits made, thought summaries).

Additional information

_No response_