fix: render permission and question prompts from nested subagent session

[ro-hansolo]

What does this PR do?

Fixes #13715

Permission and question prompts from nested subagent sessions (grandchild+) silently hang. The children() memo in session/index.tsx only collects direct child sessions, so events from deeper sessions are never rendered in the TUI.

Added a descendants() memo that walks the full session tree using Set iteration. Permissions and questions now use descendants() while tab navigation still uses the original children().

How did you verify your code works?

Spawned a general subagent that spawns another general subagent requiring bash permission. Before: TUI hangs silently. After: permission prompt surfaces. Video attached.

Before:

Screen.Recording.2026-02-15.at.3.58.30.PM.mov

After:

Bug.Fix.mov

[github-actions]

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

I found a potentially related PR:

PR #10539: fix(questions): fixes #7654 where sub-sub agents can't ask questions
#10539

This PR may be related because it also addresses an issue with nested subagents (sub-sub agents) not being able to ask questions. Your PR #13719 fixes a similar problem where permission and question prompts from nested subagent sessions were silently hanging. Both involve rendering prompts from deeper levels in the session hierarchy.

However, this appears to be a historical PR that may have already been merged. The current PR (#13719) likely addresses a regression or a similar issue with a different implementation approach.

[markwellis]

This PR solves a big problem I've been experiencing because I have most permissions set to ask. Any chance of getting it merged? Thanks