Background task management for codex - seeking approval to submit PR

Hey team! I've built a feature that lets AI agents run background tasks so they can do multiple things at once, and wanted to get approval before submitting a PR (per the contributing guidelines).

The problem this solves

Right now when the AI runs a command, it blocks until that command finishes. This is a problem for:

• Starting dev servers (blocks the AI from making code changes while server runs)
• Running long builds while continuing to work on other tasks
• Monitoring logs while deploying
• Running tests in background while writing more code

What I built

A background task management system that allows the AI to:

• Start shell commands in the background (e.g., npm run dev keeps running while AI does other work)
• Check output from background tasks anytime using bash_output tool
• Kill background tasks when done using kill_shell tool
• View all active background tasks via TUI (Ctrl+B shows list of running tasks)

Implementation details

• Built on Tokio for async process management
• Resource limits: max 20 concurrent tasks, 10k line output buffer per task
• Clean process cleanup when task/session ends
• Full test coverage (12 tests)
• Hex-based shell IDs for easy reference

This is related to #3968 - enables the same workflow where AI can start long-running processes and continue working.

Would you be open to reviewing a PR for this?

Thanks!