fix: address codex review round 1

- 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.

docs/cordis-catalog/services.md

@@ -1997,11 +1997,15 @@ async drainContinuableDescendants(parents: readonly Agent[]): Promise<void>
  * live-preferred corpus without loading or resuming an Agent. The lineage
  * trace supplies stable candidate order and live status; each candidate is
  * then inspected independently for exactly one supported descriptor in its
- * own suffix.
+ * own suffix. Session-query reads take no signal, so cancellation is
+ * cooperative: the scan rechecks `signal` after every un-signalled await and
+ * stops between candidates instead of draining a slow or large catalog after
+ * the caller has gone.
  * @param parentSessionId - parent whose direct children are listed.
+ * @param signal - caller-owned cancellation observed between query awaits.
  * @returns child and diagnostic entries in lineage-trace order.
  */
-async listChildren(parentSessionId: SessionId): Promise<SubagentListEntry[]>
+async listChildren(parentSessionId: SessionId, signal?: AbortSignal): Promise<SubagentListEntry[]>
 
 /**
  * Register a provider under its name. Registration is effect-scoped and HMR