Function register_agent 

pub fn register_agent(
    conn: &Connection,
    agent_id: &str,
    agent_type: &str,
    capabilities: &[String],
) -> Result<String>

Register or refresh an agent in the reserved _agents namespace.

Each agent is stored as a long-tier memory with title = "agent:<agent_id>". Duplicate registration for the same agent_id refreshes last_seen_at and overwrites agent_type + capabilities, while preserving the original registered_at timestamp (caller-observable provenance).

Returns the stored memory ID.