fix(tui): sort child sessions by created time, not session ID

[Zagoshipda]

Issue for this PR

Closes #42905

Type of change

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

What does this PR do?

session.child.first (Ctrl+X+Down) and the Left/Right child cycle sorted sibling sessions by session ID string. Session IDs are descending-encoded timestamps that wrap around at 2^48, so once the encoding wraps the ID order no longer matches creation order and "first child" lands on an old subagent instead of the newest.

The fix sorts children() in packages/tui/src/routes/session/index.tsx by time.created descending instead of by session ID. This is the ordering the ID sort was already encoding — "first child" keeps meaning newest-created, but is now stable across the wrap. The footer (N of M) counter already sorts by time.created ascending, so navigation and the counter stay consistent (Ctrl+X+Down lands on M of M).

How did you verify your code works?

• Reproduced the wrap: session IDs are ~current where current = timestamp_ms * 0x1000 + counter, truncated to 48 bits. The last wrap occurred at 2026-08-14 11:19:55 UTC.
• Checked against real session data: in an 829-child session spanning the wrap, the smallest-ID child was created minutes before the wrap while the actual newest child ranked ~756th in ID order; the new sort puts the newest-created child first.
• bun typecheck passes in packages/tui.

Screenshots / recordings

Not included — this changes which session navigation lands on, not how the UI looks, so there is nothing visual to capture.

Checklist

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

[github-actions]

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

Potential duplicate PR found:

• fix(tui): redo and child session ordering across id wrap #42590: fix(tui): redo and child session ordering across id wrap
  fix(tui): redo and child session ordering across id wrap #42590

  Why it's related: This PR also addresses child session ordering issues related to the timestamp ID wrap that occurs at 2^48. Both PRs are fixing the same underlying problem with session ID encoding and creation time ordering in the TUI navigation.

[github-actions]

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