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.
Rewrites the service API table, authority-versus-provenance contract, residency
routing, and deferred-work list; scopes the in-process driver README to one-shot
runs; and restates both model-facing tools' outputs, which no longer carry a
task id.
