Defer managed worktree transitions to fresh TUI loop iterations

[copyberry]

Defer managed worktree transitions to fresh TUI loop iterations

Why

Managed worktree transitions run through the event handler into the large synchronous ChatWidget constructor. Split this work so each stage runs on a fresh event-loop stack.

What changed

• Queue worktree setup, checkout completion and configuration loading, session transition, and widget attachment as separate stages with boxed futures.
• Recheck the source session before transitioning and report retained checkout cleanup instructions when the transition cannot proceed.
• Keep the old configuration paired with the old widget until attachment, and protect input during the handoff.

Testing

Update worktree lifecycle and submission tests to drain the deferred stages explicitly and assert that checkout creation starts before completion.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}