Prepare the TUI for conversation recaps (#40696)

## What changed

- Forward terminal focus changes through the TUI event stream and handle them across interactive screens.
- Track completed turns and recap eligibility across active, resumed, and switched threads.
- Add bounded recent-conversation prompt construction and a wrapping `Conversation recap` history cell.
- Cover focus timing, repeat eligibility, restored progress, UTF-8 prompt limits, and recap rendering with unit and snapshot tests.

GitOrigin-RevId: d7542c18b2bb1e72f5906f563898645bb526255f