fix(tui): guard default agent selection

[Skyline-23]

Summary

• prevent crash when agent list is empty by deferring current selection
• ensure current agent resets when list becomes available

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found several related PRs that address similar issues with agent handling in the TUI:

Potential Related PRs:

1. fix(tui): handle undefined agent.current() to prevent crash on startup #7748 - fix(tui): handle undefined agent.current() to prevent crash on startup

   • fix(tui): handle undefined agent.current() to prevent crash on startup #7748
   • Addresses similar crash prevention when agent.current() is undefined

2. fix(tui): guard local.agent.current() after oauth redirect (fixes #7727) #7747 - fix(tui): guard local.agent.current() after oauth redirect (fixes #7727)

   • fix(tui): guard local.agent.current() after oauth redirect (fixes #7727) #7747
   • Guards agent.current() in similar context (oauth redirect)

3. fix(tui): handle undefined agent.current() to prevent fatal TypeError #7689 - fix(tui): handle undefined agent.current() to prevent fatal TypeError

   • fix(tui): handle undefined agent.current() to prevent fatal TypeError #7689
   • Another related guard for undefined agent.current()

These PRs appear to be addressing the same or very similar issues around guarding against undefined/empty agent states. You may want to review them to ensure there's no overlap in the fix approach or to coordinate the solutions.