feat(cli): integrate conductor as a built-in extension

- Introduce support for bundled 'builtin' extensions in ExtensionManager
- Migrate conductor extension to packages/core/src/extensions/builtin/conductor
- Add automatic migration logic to cleanup manual conductor installations
- Make extension versioning optional for built-ins, defaulting to CLI version
- Update build and bundle scripts to include builtin extensions
- Enhance documentation to highlight conductor as a core feature

scripts/copy_bundle_assets.js

@@ -73,6 +73,17 @@ if (existsSync(builtinSkillsSrc)) {
   console.log('Copied built-in skills to bundle/builtin/');
 }
 
+// 4.1 Copy Built-in Extensions (packages/core/src/extensions/builtin)
+const builtinExtensionsSrc = join(root, 'packages/core/src/extensions/builtin');
+const builtinExtensionsDest = join(bundleDir, 'extensions', 'builtin');
+if (existsSync(builtinExtensionsSrc)) {
+  cpSync(builtinExtensionsSrc, builtinExtensionsDest, {
+    recursive: true,
+    dereference: true,
+  });
+  console.log('Copied built-in extensions to bundle/extensions/builtin/');
+}
+
 // 5. Copy DevTools package so the external dynamic import resolves at runtime
 const devtoolsSrc = join(root, 'packages/devtools');
 const devtoolsDest = join(