Implement recursive agent delegation through delegate_task tool calls

Normalized profile

What this task asks for

Run delegated sub-agents, feed their results back as tool_result, and handle unknown agents, failures, and circular delegation. Trigger delegation through `delegate_task` with an `agent_id` and delegated instructions. Run the selected sub-agent and feed one JSON `tool_result` containing its accumulated text back to the delegating agent. Match the streamed tool-use ID to `tool_result.tool_use_id`. Report unknown agents with both a stream error and an error tool result that names the requested agent. Report sub-agent failures only through an error tool result. Report circular delegation with a stream-level error whose message mentions `circular`.