ci: reaper for WS_ARCHIVE_CACHE_ROOT (~541 MB per run, nothing deletes old archives) #5913

Triage nit on #5878: the LAN archive cache under vars.WS_ARCHIVE_CACHE_ROOT (ci/sections.yml, scripts/ci/archive_fetch.sh) gains ~541 MB per run and no job removes old entries. Needed: a reaper keyed on age/sha, run by the fleet (not a PR job), with a self-test row that proves it deletes only entries older than the window and never the entry a running job points at. Refs #5878.