fix(tui): align child transcript messages with Pi

[tt-a1i]

Summary

• render child user, assistant, and thinking content with Pi native message components
• share the exact collapsed tool activity shell between parent tools and persisted child transcripts
• align queued steering/follow-up labels while preserving bounded legacy transcript fallbacks

Validation

• bun run check
• bun run test (887 Node tests, 30 Vitest tests)
• isolated real Pi TUI smoke from this checkout: Workflow child showed native user/thinking, Read, and Ran; no legacy >/~ prefixes or Transcript frame; navigation returned correctly

Closes #105