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/subagent/tool-subagent/src/index.ts

@@ -299,6 +299,7 @@ export function apply(ctx: Context, config: Config): void {
             // there, so this call neither waits for nor collects a result.
             const started = await ctx.subagents.startContinuable({
               provider: config.provider,
+              label: args.description,
               request,
               signal: exec.signal,
             })