refactor: implement singleton pattern for key managers and improve resource access

Autoloads/SaveSystem.cs

@@ -141,7 +141,7 @@ public partial class SaveSystem : Node
         var skills = new List<SkillData>();
         if (skillNames == null) return skills;
 
-        var skillManager = GetNodeOrNull<SkillManager>(Constants.SkillManagerPath);
+        var skillManager = SkillManager.Instance;
         if (skillManager == null)
         {
             GD.PrintErr("SaveSystem: SkillManager not available to resolve skill names.");