fix(tasks-local): return the layer disposer directly and cover scoped teardown

`ScopedLayers.effect` already returns an exact `() => void`, so the inherited
`() => void dispose()` wrapper voided a void — two lint rules, four errors.

The scoped layer's own teardown had no test, which is the registry-contribution
disposal contract the testing policy requires and the only path that calls
`TaskLayer.isEmpty()`: `ScopedLayers` prunes a scope's layer when its last
contribution disposes. The new case mounts one plugin contributing both a
surface and a listener into one scope, then unloads it and observes that the
agents which joined that scope are refused again.

Refs #2141

packages/tasks/tasks-local/src/index.ts

@@ -238,23 +238,21 @@ export class LocalTaskService extends TaskService {
   }
 
   onTaskDone(listener: TaskDoneListener): () => void {
-    const dispose = this.layers.effect(
+    return this.layers.effect(
       this.ctx,
       layer => layer.listeners.append(listener),
       { label: 'tasks.onTaskDone()' },
     )
-    return () => void dispose()
   }
 
   attachSurface(name: string): () => void {
     // One token per call keeps duplicate labels independently disposable.
     const token = Symbol(name)
-    const dispose = this.layers.effect(
+    return this.layers.effect(
       this.ctx,
       layer => layer.surfaces.append(token),
       { label: 'tasks.attachSurface()' },
     )
-    return () => void dispose()
   }
 
   /**