fix(opencode): collect nested subagent sessions in TUI sidebar

[StressTestor]

Issue for this PR

Closes#4422

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

children() in session/index.tsx only collected direct children of the root session. when a subagent spawns another subagent, the nested one's parentID points to the intermediate session, not the root. so it never showed up in the sidebar or keyboard navigation.

fix: added a root() memo that walks up the parent chain, then children() builds a parent->children index and BFS-collects all descendants from the root. single file change, no behavior change for non-nested cases.

How did you verify your code works?

• bun turbo typecheck passes (all 16 packages)
• full build passes (all platforms)
• traced all callers of children() (permissions, questions, sidebar visibility, keyboard nav) to confirm they handle the expanded list correctly

Screenshots / recordings

Not a UI change - logic fix only.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[rekram1-node]

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