feat: /prompts slash command (#1937)

- Shows several example prompts which include @-mentions 

------
https://chatgpt.com/codex/tasks/task_i_6894779ba8cc832ca0c871d17ee06aae

codex-rs/tui/src/chatwidget.rs

@@ -556,6 +556,10 @@ impl ChatWidget<'_> {
         ));
     }
 
+    pub(crate) fn add_prompts_output(&mut self) {
+        self.add_to_history(HistoryCell::new_prompts_output());
+    }
+
     /// Forward file-search results to the bottom pane.
     pub(crate) fn apply_file_search_result(&mut self, query: String, matches: Vec<FileMatch>) {
         self.bottom_pane.on_file_search_result(query, matches);