Phase 1 评审修复：M2M clean() 校验 + 正确页面树测试(20 tests) + AGENTS.md 同步

core/models.py

@@ -1,4 +1,5 @@
 from django.db import models
+from django.core.exceptions import ValidationError
 from django.core.validators import MinValueValidator, MaxValueValidator
 from modelcluster.fields import ParentalManyToManyField
 from wagtail.models import Page
@@ -234,6 +235,21 @@ class SkeletonProjectPage(Page):
         ),
     ]
 
+    def clean(self):
+        super().clean()
+        if self.id is not None:
+            errors = {}
+            if not self.scenarios.exists():
+                errors["scenarios"] = ValidationError(
+                    "At least one scenario is required."
+                )
+            if not self.languages.exists():
+                errors["languages"] = ValidationError(
+                    "At least one language is required."
+                )
+            if errors:
+                raise ValidationError(errors)
+
     class Meta:
         verbose_name = "Skeleton Project"