Add Ground data model for skill tree

The Ground data model stores evaluation information for each skill node.
It includes:
- The answer to be evaluated
- A list of terms that should be contained in the answer
- A list of terms that should not be contained in the answer
- A list of associated files
- A map for additional evaluation criteria

frontend/lib/models/skill_tree/ground.dart

@@ -0,0 +1,25 @@
+class Ground {
+  final String answer;
+  final List<String> shouldContain;
+  final List<String> shouldNotContain;
+  final List<String> files;
+  final Map<String, dynamic> eval;
+
+  Ground({
+    required this.answer,
+    required this.shouldContain,
+    required this.shouldNotContain,
+    required this.files,
+    required this.eval,
+  });
+
+  factory Ground.fromJson(Map<String, dynamic> json) {
+    return Ground(
+      answer: json['answer'],
+      shouldContain: List<String>.from(json['should_contain']),
+      shouldNotContain: List<String>.from(json['should_not_contain']),
+      files: List<String>.from(json['files']),
+      eval: json['eval'],
+    );
+  }
+}