helper.rs:                                                
  - Added a show_hint: Cell<bool> field to CliHelper. Uses Cell for interior mutability since hint() takes &self.                                                                                          
  - hint() now sets show_hint to false the moment line is non-empty. The hint only displays when show_hint is still true and the line is empty — i.e., only on the fresh prompt before any key is pressed. 
  - validate() calls reset_hint() after validation so the hint reappears on the next prompt after submission.                                                                                              
  - Added pub fn reset_hint(&self) for external reset.                                                                                                                                                     
                                                            
  exec.rs:
  - On Ctrl+C (ReadlineError::Interrupted), calls rl.helper().unwrap().reset_hint() so the next prompt still shows the hint.