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/lib.rs

@@ -77,6 +77,7 @@ mod style;
 mod terminal_palette;
 mod text_formatting;
 mod tooltips;
+mod transcript_copy;
 mod transcript_selection;
 mod tui;
 mod ui_consts;