Set Channel Prompts

{
  "channel_prompts": {
    "whatsapp": "Keep replies under 2 sentences.",
    "email": "Sign off as \"The Acme Team\"."
  }
}

{
  "success": true,
  "workflow_id": "123e4567-e89b-12d3-a456-426614174000",
  "channels": ["whatsapp", "email"]
}

{
  "error": "Invalid input",
  "message": "channel_prompts object is required"
}

{
  "error": "Not found",
  "message": "Workflow not found"
}