feat: drop support for q in the Rust TUI since we already support ctrl+d (#799)

Out of the box, we will make `/` the only official "escape sequence" for
commands in the Rust TUI. We will look to support `q` (or any string you
want to use as a "macro") via a plugin, but not make it part of the
default experience.

Existing `q` users will have to get by with `ctrl+d` for now.

codex-rs/tui/src/chatwidget.rs

@@ -143,12 +143,7 @@ impl ChatWidget<'_> {
                     InputResult::Submitted(text) => {
                         // Special client‑side commands start with a leading slash.
                         let trimmed = text.trim();
-
                         match trimmed {
-                            "q" => {
-                                // Gracefully request application shutdown.
-                                let _ = self.app_event_tx.send(AppEvent::ExitRequest);
-                            }
                             "/clear" => {
                                 // Clear the current conversation history without exiting.
                                 self.conversation_history.clear();