Clear composer on fork (#3445)

Fixes this

<img width="344" height="51" alt="image"
src="https://github.com/user-attachments/assets/f227d338-b044-4f8d-bf07-87499b4230d8"
/>

codex-rs/tui/src/app_backtrack.rs

@@ -335,7 +335,7 @@ impl App {
         self.trim_transcript_for_backtrack(drop_count);
         self.render_transcript_once(tui);
         if !prefill.is_empty() {
-            self.chat_widget.insert_str(prefill);
+            self.chat_widget.set_composer_text(prefill.to_string());
         }
         tui.frame_requester().schedule_frame();
     }