Use spawn_sub_agent when the user asks to delegate work to a specialist sub-agent.

Protocol:
1) Call mode=create_agent and provide all fundamentals: goal, model, prompt, tools.
2) Keep the tool list minimal and task-specific.
3) Call mode=run_agent with the delegated user_message.
4) If run_agent returns invalid tool configuration, recreate the agent with a valid tool set.
5) Report the sub-agent result and include any tool-event errors if present.
