Add an optional model for each prompt in config.yaml

[ShellOliver]

The prompts are very powerful, but would be even better if we could set a specific model for them.

Reason:
After a prompt gets refined, it works well with specific models, being able to select the model would be helpful for the usability so I dont need to remember the best model for the given prompt.

It would also be necessary to add the used model name if different to be close to the used prompt in the chat:
maybe turn to reapply - <model name>

Settings example with model: <model name from config.yaml>:

- name: explain
    description: Explain what is in context
    model: <model name from config.yaml>
    prompt: >
      Bring a short explanation of this selected code in the given context. I can ask for more details if I need to.