## Summary
This fixes a UX bug (https://github.com/openai/codex/issues/10442) where
the **"Implement this plan?"** prompt could reappear after switching
agents with `/agent` and then switching back to the original agent
during plan execution.
## Root Cause
On thread switch, the TUI rebuilds `ChatWidget`, replays buffered thread
events, then drains any queued live events.
In this flow, a `TurnComplete` can be handled twice for the same logical
turn:
1. replayed (`from_replay = true`)
2. then live (`from_replay = false`)
`ChatWidget` used `saw_plan_item_this_turn` to decide whether to show
the plan implementation prompt, but that flag was only reset on
`TurnStarted`.
If duplicate completion events occurred, stale `saw_plan_item_this_turn
= true` could cause the prompt to re-trigger unexpectedly.
## Fix
- Clear `saw_plan_item_this_turn` at the end of `on_task_complete`,
after prompt gating runs.
- This keeps the flag truly turn-scoped and prevents duplicate
`TurnComplete` handling from reopening the prompt.
Charley Cunningham
1096d6453c