fix(web): stop leaking ask_user_question into every preset

`ui-question`'s node half called `ctx.tools.register` on the host context.
`ScopedLayers.merge()` combines the global layer with the agent's exact-scope
layer, and an unscoped registration lands in the global one — so the tool
reached every agent no matter which preset composed it. `core-web`, sold as a
two-tool benchmark surface, really presented three.

Rendering a question is a host UI capability; having the tool is an agent
capability, and only a preset decides that. The node half is now empty and the
`tool-ask-user` row moved into the preset that wants it. The TUI keeps its own
row, having no presets.

The composition tests now assert the global tool layer is EMPTY, which is the
invariant that would have caught this: any tool outside a preset reaches every
agent. The browser lane's composition, seeded-history, and hermetic-skill
assertions address their registries through a composed agent for the same
reason — those services are per session now, and the host cannot resolve an
`isolate` realm by name.

packages/client/ui-question/README.zh.md

@@ -2,7 +2,7 @@
 
 [English](README.md) | 中文
 
-Web `ask_user_question` 功能插件。只有选择 Web 功能时，其主机侧才会挂载 `dsh-tool-ask-user`；浏览器侧会把 `question` 配置项注册到会话拥有的 `conversation.composer` 键控 slot 中。
+Web 提问功能插件：其浏览器侧把 `question` 配置项注册到会话拥有的 `conversation.composer` 键控 slot 中。其主机侧刻意为空——在那里挂载 `dsh-tool-ask-user` 会把工具放进注册表的**全局层**，而全局层会并入每一个 agent，无论它由哪个 preset 组装，于是一个"两工具"的 benchmark preset 实际会呈现三个。渲染提问是宿主的 UI 能力，拥有该工具则是 agent 的能力，因此 `tool-ask-user` 行属于需要它的各个 preset（以及没有 preset 的 TUI 组装）。
 
 组件每次渲染一个问题，提供进度导航、单选和多选选项、由标签后缀派生的推荐徽标，以及自定义答案。用户打开或编辑自定义答案时，多选题草稿会保留已选中的标签，因此提交项可以同时携带 `selected` 与 `custom`；单选题的自定义答案仍保持互斥。问题详情复用助手输出的 `MarkdownText` 原语，包括其 GFM 渲染与不受信任内容策略。限高卡片保持标题、导航与提交动作固定，超长的详情与选项共享内部滚动区。单选选项会立即前进；所有问题均已回答或跳过后，Enter 会提交；IME 输入法组合期间按 Enter 只会确认输入候选，不会前进。组件为整个请求提交一批结构化答案：「跳过此问题」会保留其他草稿，并为该项发出既有的空 `{ selected: [] }` 形状；关闭则以 `ASK_CANCELLED` 拒绝整个等待。