delegate(agent_id, task) hands off a task to a named agent and blocks
until the result is ready. The target agent runs with its own config
via the TargetAgentID mechanism in SubTurnConfig.
Key behaviors:
- Self-delegation explicitly rejected
- Permission gated by subagents.allow_agents (D4)
- Spawner errors preserve the underlying error via WithError
- Nil result from spawner handled gracefully
- Response attributed with target agent ID
Ref: #2148
xiaoen