Enhance CLI Help and Input Validation

- Improve the CLI by adding:
    - Validation for user inputs (e.g., ensuring todo_path is writable or valid).
    - Detailed subcommands or structured help output for future extensibility.
- Example improvement:
    - Subcommands like scan or update for more granular control:
   
      ```
      rusty-todo-md scan --todo-path ./TODO.md
      rusty-todo-md update --todo-path ./TODO.md
      ```