Function fork_from 

fn fork_from(
    input: &GateInput<'_>,
    resumable_at: &impl Fn(&str, &str, &str) -> bool,
) -> Option<ResumeGate>

ResumeGate::ForkFrom when this agent has no chain and the agent it was forked from is at the candidate: a Claude session, under the spawn’s own identity (both known, I2), whose file holds a complete record where it was recorded.