snapshot create read --keep-newer-than only in the post-backup prune,
so a value the duration parser rejects failed the run after the
snapshot was uploaded: a nightly cron line added a snapshot each night,
reported it as failed, and never deleted any. Without --prune the flag
was ignored. The command now checks the flag before it starts and fails
on an invalid duration or a missing --prune.
A failed post-backup prune is now wrapped in ErrPostBackupPruneFailed
and reported as "Backup completed, but post-backup prune failed: ..."
rather than as a failed snapshot creation. The exit status stays 1.
Model: opus-5-5
