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.

examples/acp-agent/tests/snapshots/session-query-spill/tool-schemas.expected.json

@@ -115,6 +115,14 @@
         "properties": {}
       }
     },
+    {
+      "name": "list_agents",
+      "description": "List your background subagents: every subagent you started that can receive `send_message`, whether it is still working (running) or has finished its current turn (complete — a follow-up message starts a new turn on the same conversation). Children that could not be read are reported as diagnostics instead of being silently dropped.",
+      "parameters": {
+        "type": "object",
+        "properties": {}
+      }
+    },
     {
       "name": "ralph",
       "description": "Run a foreground fresh-agent Ralph loop toward one immutable objective. Use only when the direct human explicitly asks for Ralph or fresh-agent iteration. Each round opens a new child with no parent conversation or prior child session; the shared workspace is long-term memory, and only a bounded structured report crosses rounds. The call returns when a worker reports completion or a concrete blocker, or at the round limit. Ordinary long-running same-session work belongs to goal tools.",