Prune legacy zstd archive names

README.md

@@ -11,7 +11,8 @@ logic remains common and can be updated through Git.
 1. `stages/10-containers.sh` — stop, archive, pull, start and verify projects.
 2. `methods/20-rclone.sh` — copy archives to an rclone destination when enabled.
 3. `methods/30-smb.sh` — copy archives to a mounted SMB share when enabled.
-4. `stages/90-prune.sh` — retain the newest configured number locally.
+4. `stages/90-prune.sh` — retain the newest configured number of `.zst`
+   archives locally, including legacy `.zst` and current `.tar.zst` names.
 
 Every stage can also be run directly. The old `docker-backup.sh` name invokes
 the container archive stage, while `dockerbackup_prune.sh` invokes the local