Record last use by touching the archive so the retention policy can
expire by access time, and run prune.sh after every restore (even on
miss) to keep the shared store trimmed: keep the 3 newest archives per
key-prefix family, expire anything older than 7 days, hard-cap the store
at 2 GB (oldest first, trumping the keep set), never touch files younger
than 1h, and drop stale run workspaces under ~/.cache/act older than 2
days.
