Do not prune after a backup that found a volume not mounted (closes #395)

snapshot create --prune purged the older snapshots holding a volume's
files when the volume, mounted below a configured path, was not
mounted: its empty mount point read as deleted files.

The scan now records in the local index each directory at or below a
configured path that is on a different device from its parent
directory. Under --prune, a run that does not find such a directory
recorded by an earlier snapshot of that name skips the prune, keeps
every older snapshot, names the directory and its snapshot, and exits
non-zero, as for an empty configured path. A volume retired on purpose
is cleared by removing the older snapshots with snapshot purge.

Model: opus-5-5

internal/cli/snapshot.go

@@ -116,7 +116,8 @@ on Linux), then /etc/vaultik/config.yml.`,
 				}
 
 				if errors.Is(err, vaultik.ErrFilesUnreadable) ||
-					errors.Is(err, vaultik.ErrSourcePathEmpty) {
+					errors.Is(err, vaultik.ErrSourcePathEmpty) ||
+					errors.Is(err, vaultik.ErrVolumeNotMounted) {
 					log.Error("Backup completed with an error", "error", err)
 					ReportErrorf("Backup completed, but %v", err)
 
@@ -137,7 +138,8 @@ on Linux), then /etc/vaultik/config.yml.`,
 			"directory under metadata/ with no manifest once all its "+
 			"objects are older than prune_grace_period and its snapshot is "+
 			"not in the local index (not after a backup that skipped "+
-			"unreadable files or found a configured path empty)")
+			"unreadable files, found a configured path empty, or found "+
+			"no volume where an earlier snapshot found one mounted)")
 	cmd.Flags().StringVar(&opts.KeepNewerThan, "keep-newer-than", "",
 		"With --prune: keep snapshots newer than this duration "+
 			"(e.g. 30d, 4w, 6mo, 1y; m is minutes, mo is months) "+