Function fold_monitor 

fn fold_monitor(
    domain: &mut DomainSession,
    msg: &Message,
    origin: MonitorOrigin,
)

Fold one wire message into the session’s monitor set: the Monitor tool call that starts one, the task id the CLI assigns it, and the lifecycle message that settles it.

The CLI’s task ids are what the terminal transitions are keyed by, so a record’s task_id is what lets a later task_updated find it at all.