feat: optional prompt only available for plan-phase

As an engineer I would like to have the option to guide the AI in every step of the modernize-cli by providing a custom prompt (not only in the planning phase).

<img width="496" height="142" alt="Image" src="https://github.com/user-attachments/assets/99559e52-4553-4dc1-9f04-ba62244d5a30" />