feat: Add Python class inheritance extraction for superclass relationships

Addresses Python's class definition syntax where parent classes are specified in argument_list nodes (e.g. `class Child(Parent, Mixin):`). Extracts identifier and attribute children from argument_list as 'extends' references to properly model Python's inheritance patterns in the code graph.

test_python_inheritance.js

@@ -0,0 +1,35 @@
+const { extractFromSource } = require('./dist/extraction');
+const { initGrammars, loadAllGrammars } = require('./dist/extraction/grammars');
+
+(async () => {
+  await initGrammars();
+  await loadAllGrammars();
+
+  const code = `
+class Parent:
+    pass
+
+class Child(Parent):
+    pass
+
+class Multiple(Parent, Mixin):
+    pass
+`;
+
+  const result = extractFromSource('test.py', code);
+
+  console.log('=== NODES ===');
+  result.nodes.forEach(n => {
+    console.log(`${n.kind}: ${n.name} (line ${n.startLine})`);
+  });
+
+  console.log('\n=== UNRESOLVED REFERENCES ===');
+  result.unresolvedReferences.forEach(r => {
+    console.log(`${r.referenceKind}: ${r.referenceName} (from ${r.fromNodeId})`);
+  });
+
+  console.log('\n=== EDGES ===');
+  result.edges.forEach(e => {
+    console.log(`${e.kind}: ${e.source} -> ${e.target}`);
+  });
+})();