feat(tui2): add copy selection shortcut + UI affordance (#8462)

- Detect Ctrl+Shift+C vs VS Code Ctrl+Y and surface in footer hints
- Render clickable “⧉ copy” pill near transcript selection (hidden while
dragging)
- Handle copy hotkey + click to copy selection
- Document updated copy UX

VSCode:
<img width="1095" height="413" alt="image"
src="https://github.com/user-attachments/assets/84be0c82-4762-4c3e-80a4-c751c078bdaa"
/>

Ghosty:
<img width="505" height="68" alt="image"
src="https://github.com/user-attachments/assets/109cc1a1-f029-4f7e-a141-4c6ed2da7338"
/>

codex-rs/tui2/src/chatwidget.rs

@@ -3096,9 +3096,14 @@ impl ChatWidget {
         scrolled: bool,
         selection_active: bool,
         scroll_position: Option<(usize, usize)>,
+        copy_selection_key: crate::key_hint::KeyBinding,
     ) {
-        self.bottom_pane
-            .set_transcript_ui_state(scrolled, selection_active, scroll_position);
+        self.bottom_pane.set_transcript_ui_state(
+            scrolled,
+            selection_active,
+            scroll_position,
+            copy_selection_key,
+        );
     }
 
     /// Forward an `Op` directly to codex.