Move restore to snapshot restore subcommand

Renames the top-level `restore` command to `vaultik snapshot restore`
for consistency with `vaultik snapshot create`. The factory follows the
sibling pattern (newSnapshotRestoreCommand) and its file is renamed to
snapshot_restore.go to match.

internal/cli/snapshot.go

@@ -27,6 +27,7 @@ func NewSnapshotCommand() *cobra.Command {
 	cmd.AddCommand(newSnapshotRemoveCommand())
 	cmd.AddCommand(newSnapshotPruneCommand())
 	cmd.AddCommand(newSnapshotCleanupCommand())
+	cmd.AddCommand(newSnapshotRestoreCommand())
 
 	return cmd
 }