feat(subagent): durable child catalog and list_agents

Implements the durable-subagent-catalog RFC: SubagentControlService.listChildren()
enumerates a parent's direct continuable children from one sessionQuery trace,
validates each child's sole subagent/descriptor event (now carrying the durable
creation label), and returns one ordered SubagentListEntry[] with per-child
corrupt/unsupported/unavailable diagnostics. The list_agents tool ships as a
separately loadable plugin of dsh-tool-subagent-control requiring sessionQuery
at load; send_message stays usable without it.

packages/workspace/workspace/tsconfig.json

@@ -32,6 +32,9 @@
     {
       "path": "../../util/brand"
     },
+    {
+      "path": "../../session-query/session-query"
+    },
     {
       "path": "../../support/invariants"
     }