Function bind_agent_did 

pub fn bind_agent_did(principal_did: &str, session_id: &str) -> String

Derive the deterministic sub-agent DID under which an agent acts in a session, scoped to principal_did + session_id.

A thin wrapper over crate::chat_agents::compile_sub_agent_did — the same derivation the chat layer uses, so a roster agent and its in-session sub-agent share one DID. Writing the session’s agent_config.json is the caller’s responsibility; this only computes the DID.