Add a UI hint when you press @ (#1903)

This will make @ more discoverable (even though it is currently not
super useful, IMO it should be used to bring files into context from
outside CWD)

---------

Co-authored-by: Gabriel Peal <gpeal@users.noreply.github.com>

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

@@ -54,6 +54,17 @@ impl FileSearchPopup {
         }
     }
 
+    /// Put the popup into an "idle" state used for an empty query (just "@").
+    /// Shows a hint instead of matches until the user types more characters.
+    pub(crate) fn set_empty_prompt(&mut self) {
+        self.display_query.clear();
+        self.pending_query.clear();
+        self.waiting = false;
+        self.matches.clear();
+        // Reset selection/scroll state when showing the empty prompt.
+        self.state.reset();
+    }
+
     /// Replace matches when a `FileSearchResult` arrives.
     /// Replace matches. Only applied when `query` matches `pending_query`.
     pub(crate) fn set_matches(&mut self, query: &str, matches: Vec<FileMatch>) {