插件删除回调（onRemove）与模板/示例同步

- sdk: RegisterOnRemoveHandler/RunOnRemoveHandlers（仅卸载时触发，重载不触发；
  后注册先执行、幂等），与 RegisterStopHandler/RunStopHandlers 并存
- plugindev: 模板 main.go.tmpl 新增 onRemove 演示（删配置键），templates.go 同步
- example/calendar: RegisterOnRemoveHandler(p.cleanupData) 卸载时清理 events.json
- README/README_EN: 生命周期文档补充 onRemove
- example/calendar/plg.json: 版本对齐

README_EN.md

@@ -208,6 +208,7 @@ Supports both **Go** and **Lua** plugin languages.
 
 - `Start(sdk *PluginSDK) error` — Plugin startup, receives SDK instance
 - `Stop() error` — Plugin shutdown, release resources
+- `sdk.RegisterStopHandler(fn func())` — Register a shutdown cleanup callback. The kernel (for built-in plugins) or z_bridge (for external plugins) runs all registered handlers **before** calling the plugin's `Stop()` (LIFO order, cleared after running — idempotent). Use it for persistence and cancelling background work: plugin memory is still fresh at that point, avoiding stale-state write-backs that resurrect deleted data.
 
 ### Auto-Restart