fix(subagent): keep one-shot labels optional

packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts

@@ -49,16 +49,8 @@ function text(blocks: { type: string; text?: string }[]): string {
   return blocks.filter(b => b.type === 'text').map(b => b.text).join('')
 }
 
-function start(
-  ctx: Context,
-  provider: string,
-  request: Omit<SubagentStartRequest, 'label' | 'signal'> & { label?: string; signal?: AbortSignal },
-) {
-  return ctx.subagents.start(provider, {
-    label: request.label ?? 'child task',
-    signal: request.signal ?? new AbortController().signal,
-    ...request,
-  })
+function start(ctx: Context, provider: string, request: Omit<SubagentStartRequest, 'signal'> & { signal?: AbortSignal }) {
+  return ctx.subagents.start(provider, { signal: request.signal ?? new AbortController().signal, ...request })
 }
 
 /** Invoke the child lifecycle effect while its parent-owned setup is still unpublished. */