Add --keep-newer-than flag for rolling retention window

snapshot create --prune now accepts --keep-newer-than <duration> (e.g.
4w, 30d, 6mo) to keep a rolling window of snapshots instead of only
the latest. Supports d/w/mo/y units and combinations (2w3d).

Without --keep-newer-than, --prune still defaults to keep-latest-only.

internal/cli/snapshot.go

@@ -101,6 +101,7 @@ specifying a path using --config or by setting VAULTIK_CONFIG to a path.`,
 
 	cmd.Flags().BoolVar(&opts.Cron, "cron", false, "Run in cron mode (silent unless error)")
 	cmd.Flags().BoolVar(&opts.Prune, "prune", false, "After backup, drop older snapshots of the same name and remove orphaned blobs")
+	cmd.Flags().StringVar(&opts.KeepNewerThan, "keep-newer-than", "", "With --prune: keep snapshots newer than this duration (e.g. 4w, 30d, 6mo) instead of only the latest")
 	cmd.Flags().BoolVar(&opts.SkipErrors, "skip-errors", false, "Skip file read errors (log them loudly but continue)")
 
 	return cmd