Gate review: `--keep` pruning ran after the summary regardless of `errors`,
so a timer hitting the same unreadable file every run would exit 1 each time
and still rotate every complete `hermes-backup-*.zip` out after N runs,
leaving only incomplete archives. Prune only after a complete backup; the
test pins a pre-existing good archive surviving an incomplete run with
`--keep 1`. The summary no longer hard-codes the caller's exit code.
1250a3e4eb