Modifies an existing VYBR itinerary based on user refinement requests. Use when user says "make day X more relaxed", "add more food options", "replace morning activity", "we're tired", or any modification to an existing plan. Returns the same JSON schema as generating-itineraries, preserving unchanged sections.
Return the complete itinerary JSON with the same schema as generating-itineraries. Only the modified sections should have changed content — all other fields remain identical.