add --pidfile argument

What variant of Codex are you using?

CLI

What feature would you like to see?

I am writing an i3 bar module that tracks running codex instances and I would like to support "jump to window on click"

right now I have to do a bit of gymnastics to map codex PIDs to thread ids: have shim that fires on notify that discovers the calling codex's pid and binds that to the thread ID payload. this is janky and doesn't work out of the box.

I'd love to be able to pass --pidfile that would just write <thread_id>.codex.pid to $XDG_STATE_HOME/

This is a bit of a niche feature, but it also seems simple, so I hope you'll take it up for consideration.

Additional information

_No response_