When the user selected a custom agent (e.g. Codex) and chose "Start
Thread in New Worktree", the new workspace's panel ignored the selection
and defaulted to Zed Agent (NativeAgent).

The fix captures `self.selected_agent()` in
`handle_worktree_creation_requested` before the async task, passes it
through to `setup_new_workspace`, and forwards it to `external_thread`
instead of `None`.

Closes AI-84

(No release notes because it's still feature-flagged.)

Release Notes:

- N/A