SubagentRunStream

A nested named-agent execution surfaced on run.subagents.

A subagent is a nested createAgent run whose lc_agent_name differs from its parent's — e.g. a createAgent({ name }) invoked inside a tool body. The handle exposes scoped projections (messages, toolCalls, nested subagents) plus the resolved name, the cause that triggered it, and the subagent's final output state.

This is a self-contained handle (no subclass of SubgraphRunStream): its projections are produced by per-subagent transformer instances that createSubagentTransformer drives directly.