Add CLAUDE.md for project guidance and enhance get_object_subtree_with_fields to handle circular references

- Introduced CLAUDE.md to provide comprehensive guidance on the Penpot MCP Server, including project overview, key commands, architecture, and common workflows.
- Enhanced the `get_object_subtree_with_fields` function to track visited nodes and handle circular references, ensuring robust tree structure retrieval.
- Added tests for circular reference handling in `test_penpot_tree.py` to validate new functionality.

tests/test_penpot_tree.py

@@ -1087,4 +1087,61 @@ def test_get_object_subtree_with_fields_root_frame():
     assert result['tree']['type'] == 'frame'
     assert 'children' in result['tree']
     assert len(result['tree']['children']) == 1
-    assert result['tree']['children'][0]['name'] == 'Main Container' 
+    assert result['tree']['children'][0]['name'] == 'Main Container'
+
+
+def test_get_object_subtree_with_fields_circular_reference():
+    """Test handling of circular references in object tree."""
+    file_data = {
+        'data': {
+            'pagesIndex': {
+                'page1': {
+                    'name': 'Test Page',
+                    'objects': {
+                        # Object A references B as parent
+                        'object-a': {
+                            'type': 'frame',
+                            'name': 'Object A',
+                            'parentId': 'object-b'
+                        },
+                        # Object B references A as parent (circular)
+                        'object-b': {
+                            'type': 'frame',
+                            'name': 'Object B',
+                            'parentId': 'object-a'
+                        },
+                        # Object C references itself as parent
+                        'object-c': {
+                            'type': 'frame',
+                            'name': 'Object C',
+                            'parentId': 'object-c'
+                        }
+                    }
+                }
+            }
+        }
+    }
+    
+    # Test getting object A - should handle circular reference with B
+    result = get_object_subtree_with_fields(file_data, 'object-a')
+    assert 'error' not in result
+    assert result['tree']['id'] == 'object-a'
+    assert 'children' in result['tree']
+    # Check that object-b appears as a child
+    assert len(result['tree']['children']) == 1
+    assert result['tree']['children'][0]['id'] == 'object-b'
+    # The circular reference appears when object-a appears again as a child of object-b
+    assert 'children' in result['tree']['children'][0]
+    assert len(result['tree']['children'][0]['children']) == 1
+    assert result['tree']['children'][0]['children'][0]['id'] == 'object-a'
+    assert result['tree']['children'][0]['children'][0]['_circular_reference'] == True
+    
+    # Test getting object C - should handle self-reference
+    result = get_object_subtree_with_fields(file_data, 'object-c')
+    assert 'error' not in result
+    assert result['tree']['id'] == 'object-c'
+    assert 'children' in result['tree']
+    # Check that object-c appears as its own child with circular reference marker
+    assert len(result['tree']['children']) == 1
+    assert result['tree']['children'][0]['id'] == 'object-c'
+    assert result['tree']['children'][0]['_circular_reference'] == True