- listChildren() takes an optional AbortSignal and rechecks it after every
un-signalled session-query await (the cold-resume cooperative-cancellation
boundary); list_agents forwards exec.signal so the registry's drain of
started tool bodies cannot block on a slow or large catalog.
- The list_agents description now presents running/complete as a stored-record
snapshot and defers deliverability to send_message, matching the ownership-
conflict semantics the service tests pin.