feat(tui): add Alt+C keyboard shortcut to copy last response

Adds Alt+C as a keybinding that triggers the same copy-to-clipboard
logic as the /copy slash command, making it quicker to grab the last
agent response as raw markdown without typing a command.

Author: AnttiJalomaki

codex-rs/tui/src/chatwidget.rs

@@ -4778,6 +4778,18 @@ impl ChatWidget {
                 self.quit_shortcut_expires_at = None;
                 self.quit_shortcut_key = None;
             }
+            KeyEvent {
+                code: KeyCode::Char(c),
+                modifiers,
+                kind: KeyEventKind::Press,
+                ..
+            } if modifiers.contains(KeyModifiers::ALT)
+                && !modifiers.contains(KeyModifiers::CONTROL)
+                && c.eq_ignore_ascii_case(&'c') =>
+            {
+                self.dispatch_command(SlashCommand::Copy);
+                return;
+            }
             KeyEvent {
                 code: KeyCode::Char(c),
                 modifiers,