Codex app: support selecting existing worktrees.

What variant of Codex are you using?

App

What feature would you like to see?

Current behavior:
Only creating a new worktree is supported when starting a new conversation.

<img width="758" height="688" alt="Image" src="https://github.com/user-attachments/assets/f02def38-6868-4e13-b54c-08b333a3fe38" />

Issue:
Sometimes the conversation has been archived, or I want to reset the context, so I need to start a new conversation while continuing work on the same worktree.

Additional information

Codex App Version: 26.506.31421 (2620)
System: Darwin 25.4.0 arm64 arm