Function sweep_scope 

pub(crate) async fn sweep_scope(
    host: &dyn SlackHost,
    api: &dyn SlackApi,
    workspace: &str,
    scope: SweepScope,
) -> Result<SweepOutcome, SlackError>

One pass over the conversations in scope: fetch what is newer than each conversation’s watermark, follow the threads of the parents that report replies, and hand every survivor to the host.