Use model catalog collaboration mode messages

[copyberry]

Use model catalog collaboration mode messages

What changed

• Add optional default and plan collaboration-mode messages to model catalog metadata.
• Prefer the active mode's catalog message, including an explicit empty value, while falling back to the existing developer instructions when the catalog variant is absent.
• Track the model in collaboration-mode world state so mode and model changes append the appropriate instructions, and clear prior instructions when no current message exists.
• Preserve catalog collaboration messages when instruction overrides remove other model instruction fields.

Testing

• Cover catalog deserialization, variant selection, fallback and empty-message behavior, legacy snapshot migration, mode changes, and model changes.