interface SubagentRunStreamA 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.
Error name for instanceof checks and logging
Combined stdout and stderr output of the executed command
A list of additional subagents to provide to the agent