feat: add undo

app/src/main/java/chess/controller/commands/UndoCommand.java

@@ -2,14 +2,18 @@ package chess.controller.commands;
 
 import chess.controller.Command;
 import chess.controller.OutputSystem;
+import chess.controller.PlayerCommand;
 import chess.model.Game;
 
 public class UndoCommand extends Command{
 
 	@Override
 	public CommandResult execute(Game game, OutputSystem outputSystem) {
-		//TODO
-		return CommandResult.Moved;
+		PlayerCommand lastAction = game.getLastAction();
+		if (lastAction == null) 
+			return CommandResult.NotAllowed;
+		
+		return lastAction.undo(game, outputSystem);
 	}
 	
 }