Context mode. Defaults to isolated, where the subagent only sees the delegated task.
Under fork, the subagent receives the parent's effective conversation
history and state, and mirrors the parent's prompt-producing middleware so
it rebuilds the same system prompt. It cannot define skills, which would
diverge from the parent's. tools isn't restricted the same way -- a fork's
own tools work normally; the tradeoff is cache misses.
