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/root.go

@@ -90,7 +90,6 @@ on the source system.`,
 	// Add subcommands
 	cmd.AddCommand(
 		NewConfigCommand(),
-		NewRestoreCommand(),
 		NewPruneCommand(),
 		NewStoreCommand(),
 		NewSnapshotCommand(),