Connected-agent governance: persist connections + revoke primitive

memsprout now tracks which agent (MCP client) holds which live session, one durable record per install. A new revoke_agent_connection RPC immediately kills the underlying session and stamps the record so that even an un-expired token is rejected on the next MCP call — giving admins and users a real per-agent kill switch for the first time. Applies to new connections from this release forward; existing sessions age out naturally.