Announce that an MCP client has launched with this key. Fire-and-forget; records presence, not usage.

Called automatically by @pathmode/mcp-server 1.9.0+ once per launch, on the MCP oninitialized callback. Stamps client_first_seen_at (first time) and client_seen_at, and emits exactly one event, mcp_client_launched; never touches last_used_at, because a client starting up is not an agent doing work. Body is optional and all fields are untrusted client-reported strings. Clients must treat failures as non-fatal.