TUI: Add session resume picker (--resume) and quick resume (--continue) (#3135)

Adds a TUI resume flow with an interactive picker and quick resume.

- CLI: 
  - --resume / -r: open picker to resume a prior session
  - --continue   / -l: resume the most recent session (no picker)
- Behavior on resume: initial history is replayed, welcome banner
hidden, and the first redraw is suppressed to avoid flicker.
- Implementation:
- New tui/src/resume_picker.rs (paginated listing via
RolloutRecorder::list_conversations)
  - App::run accepts ResumeSelection; resumes from disk when requested
- ChatWidget refactor with ChatWidgetInit and new_from_existing; replays
initial messages
- Tests: cover picker sorting/preview extraction and resumed-history
rendering.
- Docs: getting-started updated with flags and picker usage.



https://github.com/user-attachments/assets/1bb6469b-e5d1-42f6-bec6-b1ae6debda3b

codex-rs/tui/src/app_backtrack.rs

@@ -319,14 +319,16 @@ impl App {
     ) {
         let conv = new_conv.conversation;
         let session_configured = new_conv.session_configured;
-        self.chat_widget = crate::chatwidget::ChatWidget::new_from_existing(
-            cfg,
-            conv,
-            session_configured,
-            tui.frame_requester(),
-            self.app_event_tx.clone(),
-            self.enhanced_keys_supported,
-        );
+        let init = crate::chatwidget::ChatWidgetInit {
+            config: cfg,
+            frame_requester: tui.frame_requester(),
+            app_event_tx: self.app_event_tx.clone(),
+            initial_prompt: None,
+            initial_images: Vec::new(),
+            enhanced_keys_supported: self.enhanced_keys_supported,
+        };
+        self.chat_widget =
+            crate::chatwidget::ChatWidget::new_from_existing(init, conv, session_configured);
         // Trim transcript up to the selected user message and re-render it.
         self.trim_transcript_for_backtrack(drop_count);
         self.render_transcript_once(tui);