Function add 

pub async fn add(
    memory: Option<&Arc<MemoryService>>,
    content: impl Into<String>,
    scope: MemoryScope,
    memory_type: MemoryType,
    user_ctx: &UserContext,
    agent_id: Option<&str>,
    session_id: Option<&str>,
) -> Result<Memory>

Add a memory.

Goes through the service rather than the store because the service owns EMBEDDING: a row written without its vector is invisible to every later semantic search, which presents as data loss rather than a missing index.