Remove the Plan mode composer nudge (#40200)

## What changed

- Keep the normal composer footer visible when a draft contains the word `plan`.
- Remove the contextual “Create a plan?” prompt, its Escape dismissal behavior, and its per-thread dismissal state.

## Testing

- Update wide and narrow composer snapshots to cover the standard footer for plan-related drafts.

GitOrigin-RevId: 1a3c8cfce9d4effa2149790975133f652aa7c914

codex-rs/tui/src/chatwidget/session_flow.rs

@@ -28,7 +28,6 @@ impl ChatWidget {
             self.review.recent_auto_review_denials = RecentAutoReviewDenials::default();
             self.clear_thread_usage_state();
         }
-        self.refresh_plan_mode_nudge();
         self.turn_lifecycle.reset_thread();
         self.clear_safety_buffering();
         self.thread_name = session.thread_name.clone();
@@ -106,7 +105,6 @@ impl ChatWidget {
                     mask.reasoning_effort = Some(session.reasoning_effort.clone());
                 }
                 self.update_collaboration_mode_indicator();
-                self.refresh_plan_mode_nudge();
             }
         }
         let effort = self.effective_reasoning_effort();