Function subagent_stop_reason 

pub fn subagent_stop_reason(
    kind: &str,
    issue: Option<&str>,
    decision: bool,
    active: bool,
) -> Option<String>

The stop gate for a subagent: once, when its parent holds an issue, the reason the subagent is kept working one more round. A gate that already held it this turn, or a parent holding nothing, lets it stop.