Add new skills: Double Jump, Ground Pound, Brick Armor, and Brick Shield; implement skill removal signal in SkillManager (#11)

scripts/UI/MarketplaceButton.cs

@@ -14,6 +14,7 @@ public partial class MarketplaceButton : Button
     
     private GameManager _gameManager;
     private SkillUnlockerComponent _skillUnlockerComponent;
+    private SkillManager _skillManager;
 
     public override void _Ready()
     {
@@ -27,6 +28,9 @@ public partial class MarketplaceButton : Button
             _skillUnlockerComponent.SkillUnlocked += OnSkillStateChanged;
         }
         
+        _skillManager = GetNode<SkillManager>("/root/SkillManager");
+        _skillManager.SkillRemoved += OnSkillStateChanged;
+        
         UpdateButtonState();
     }