Function is_control_only 

fn is_control_only(plan: &Plan) -> bool

Render the in-flight forest as a system-prompt block so the LLM can see what is still running and reference a plan_id to cancel it. Empty when no tree is running. Trees are ordered by numeric plan id for stable output. True when every do node is a plan-control builtin and there is at least one. Plan-control trees are not themselves cancellable task work; advertising them makes the model inspect or cancel its own control actions.