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/key_hint.rs

@@ -53,6 +53,10 @@ pub(crate) const fn ctrl_alt(key: KeyCode) -> KeyBinding {
     KeyBinding::new(key, KeyModifiers::CONTROL.union(KeyModifiers::ALT))
 }
 
+pub(crate) const fn ctrl_shift(key: KeyCode) -> KeyBinding {
+    KeyBinding::new(key, KeyModifiers::CONTROL.union(KeyModifiers::SHIFT))
+}
+
 fn modifiers_to_string(modifiers: KeyModifiers) -> String {
     let mut result = String::new();
     if modifiers.contains(KeyModifiers::CONTROL) {