text: Title: Simplify PrepromptsHolder to just be a function
  Body: Currently preprompt holders is this:
  
  ```
  class PrepromptsHolder:
      def __init__(self, preprompts_path: Path):
          self.preprompts_path = preprompts_path
  
      def get_preprompts(self) -> Dict[str, str]:
          preprompts_repo = DiskMemory(self.preprompts_path)
          return {file_name: preprompts_repo[file_name] for file_name in preprompts_repo}
  ```
  
  We could change it to just a function:
  
  ```
  def get_preprompts(preprompts_path: Path) -> Dict[str, str]:
      return dict(DiskMemory(preprompts_path).items())
  ```
  