fix(background-agent): clear toast tracking when tasks stop

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/background-agent/remove-task-toast-tracking.ts

@@ -0,0 +1,8 @@
+import { getTaskToastManager } from "../task-toast-manager"
+
+export function removeTaskToastTracking(taskId: string): void {
+  const toastManager = getTaskToastManager()
+  if (toastManager) {
+    toastManager.removeTask(taskId)
+  }
+}