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

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[github-actions[bot]]

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

   • https://github.com/anomalyco/opencode/pull/7748
   • Addresses similar crash prevention when agent.current() is undefined

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

   • https://github.com/anomalyco/opencode/pull/7747
   • Guards agent.current() in similar context (oauth redirect)

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

   • https://github.com/anomalyco/opencode/pull/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.