add fast mode toggle (#13212)

- add a local Fast mode setting in codex-core (similar to how model id
is currently stored on disk locally)
- send `service_tier=priority` on requests when Fast is enabled
- add `/fast` in the TUI and persist it locally
- feature flag

codex-rs/tui/src/bottom_pane/mod.rs

@@ -294,6 +294,11 @@ impl BottomPane {
         self.request_redraw();
     }
 
+    pub fn set_fast_command_enabled(&mut self, enabled: bool) {
+        self.composer.set_fast_command_enabled(enabled);
+        self.request_redraw();
+    }
+
     pub fn set_realtime_conversation_enabled(&mut self, enabled: bool) {
         self.composer.set_realtime_conversation_enabled(enabled);
         self.request_redraw();