number is a per-repo/workflow counter with no meaning once the admin
endpoint mixes every repo on the instance into one feed -- a rarely-run
workflow's low #53 that just fired outranked a busier workflow's #142 from
hours earlier, but sorting by number buried it instead. Sort by started
(globally comparable) with run_id/attempt as tiebreaks; a re-run bumping
started_at back to the top is correct, not a bug, under "most recent
globally".