Resume conversation from an earlier point in history (#2607)

Fixing merge conflict of this: #2588


https://github.com/user-attachments/assets/392c7c37-cf8f-4ed6-952e-8215e8c57bc4

codex-rs/tui/src/lib.rs

@@ -25,8 +25,10 @@ use tracing_subscriber::EnvFilter;
 use tracing_subscriber::prelude::*;
 
 mod app;
+mod app_backtrack;
 mod app_event;
 mod app_event_sender;
+mod backtrack_helpers;
 mod bottom_pane;
 mod chatwidget;
 mod citation_regex;