Save a memory to the memory bank. Body: { content: string (required); tags?: string[]; contexts?: Array<{ snippet: string (required); conversation_id?: string; message_id?: string; origin?: "active_conversation" | "searched_conversation" }> }.

Persists a human-confirmed memory for the calling user. The server sets the source and always stores it as a personal ('user') memory regardless of any scope sent by the client.