feat(subagent): let ancestors interrupt descendants

interrupt_agent(agent_id) passes the calling agent as the ancestor
authority for ctx.subagents.interrupt(); the service verifies live
registry identity and recorded lineage, so a direct child or deeper
descendant stops with the same generic parameter while send_message keeps
its exact-direct-parent authority.

Discovery: list_agents gains an optional scope. descendants walks the new
SubagentService.listDescendants() — one lineage trace flattened in stable
pre-order across ordinary and one-shot intermediates, each entry carrying
its verified parentId and depth — and every status now comes from the
live Agent registry (running/idle/complete).

Refs #1535

scripts/type-equiv.manifest.json

@@ -1155,6 +1155,11 @@
       "symbol": "SubagentInterruptAuthority",
       "source": "packages/subagent/subagent/src/continuation.ts"
     },
+    {
+      "doc": "docs/core-data-structures/subagent.md",
+      "symbol": "SubagentDescendantListEntry",
+      "source": "packages/subagent/subagent/src/list-children.ts"
+    },
     {
       "doc": "docs/core-data-structures/subagent.md",
       "symbol": "ContinuableStart",