feat(subagent): catalog one-shot child sessions

packages/workflow/workflow-workerthread/src/host.ts

@@ -322,6 +322,7 @@ export class WorkerRun implements WorkflowRun {
     let run: SubagentRun
     try {
       run = await this.subagents.start(this.provider, {
+        label: request.label,
         prompt: [{ type: 'text', text: request.prompt }],
         parent: this.parent,
         signal: this.controller.signal,

packages/workflow/workflow-workerthread/src/runtime.ts

@@ -275,6 +275,7 @@ export class WorkflowExecution {
       let run: ChildHandle
       try {
         run = await this.children.startAgent({
+          label,
           prompt: rawPrompt,
           ...opts.schema !== undefined ? { schema: opts.schema } : {},
           ...opts.provider !== undefined ? { provider: opts.provider } : {},

packages/workflow/workflow-workerthread/src/types.ts

@@ -38,6 +38,8 @@ export interface WorkerInit {
 
 /** What the worker asks the host to start for one `agent()` call (options already validated worker-side). */
 export interface ChildStartRequest {
+  /** Short display label resolved by the worker runtime. */
+  label: string
   /** The child's prompt text. */
   prompt: string
   /** The structured-output schema, if the call passed one (already subset-checked). */