feat(behavior-tree): add pure BehaviorTreePlugin for Cocos/Laya integration (#400)

- Add BehaviorTreePlugin class that only depends on @esengine/ecs-framework
- Implement IPlugin interface with install(), uninstall(), setupScene() methods
- Remove esengine/ subdirectory that incorrectly depended on engine-core
- Update package documentation with correct usage examples

.changeset/add-behavior-tree-plugin.md

@@ -0,0 +1,24 @@
+---
+"@esengine/behavior-tree": minor
+---
+
+feat(behavior-tree): add pure BehaviorTreePlugin class for Cocos/Laya integration
+
+- Added `BehaviorTreePlugin` class that only depends on `@esengine/ecs-framework`
+- Implements `IPlugin` interface with `install()`, `uninstall()`, and `setupScene()` methods
+- Removed `esengine/` subdirectory that incorrectly depended on `@esengine/engine-core`
+- Updated package documentation with correct usage examples
+
+Usage:
+```typescript
+import { Core, Scene } from '@esengine/ecs-framework';
+import { BehaviorTreePlugin, BehaviorTreeBuilder, BehaviorTreeStarter } from '@esengine/behavior-tree';
+
+Core.create();
+const plugin = new BehaviorTreePlugin();
+await Core.installPlugin(plugin);
+
+const scene = new Scene();
+plugin.setupScene(scene);
+Core.setScene(scene);
+```