Delete Agent Namespace

Hard-delete an agent namespace: purge every memory + fact written under this agent_id, so a typo or throwaway agent_id stops appearing in GET /v1/agents. Soft-forgetting its data is not enough for that: the listing counts any row carrying the namespace, forgotten ones included. Returns the purge counts + an audit id. 404 if the namespace was never used in this project.

agent_id is percent-encoded as one path segment (a/b is a%2Fb); any stored id is accepted, including one written before new ids were required to be a single segment.