disallow some slash commands while a task is running (#2792)

/new, /init, /models, /approvals, etc. don't work correctly during a
turn. disable them.

codex-rs/tui/src/chatwidget.rs

@@ -751,12 +751,20 @@ impl ChatWidget {
     }
 
     fn dispatch_command(&mut self, cmd: SlashCommand) {
+        if !cmd.available_during_task() && self.bottom_pane.is_task_running() {
+            let message = format!(
+                "'/'{}' is disabled while a task is in progress.",
+                cmd.command()
+            );
+            self.add_to_history(history_cell::new_error_event(message));
+            self.request_redraw();
+            return;
+        }
         match cmd {
             SlashCommand::New => {
                 self.app_event_tx.send(AppEvent::NewSession);
             }
             SlashCommand::Init => {
-                // Guard: do not run if a task is active.
                 const INIT_PROMPT: &str = include_str!("../prompt_for_init_command.md");
                 self.submit_text_message(INIT_PROMPT.to_string());
             }