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, and a locally authored
`bash-only` preset presented two.

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 presets that want it. The TUI keeps
its own row, having no presets.

The web composition test now asserts the global tool layer is EMPTY, which
is the invariant that would have caught this: any tool outside a preset
reaches every agent.

packages/client/ui-question/README.md

@@ -2,7 +2,7 @@
 
 English | [中文](README.zh.md)
 
-Web `ask_user_question` feature plugin. Its host half mounts `dsh-tool-ask-user` only when the Web feature is selected; its browser half registers the `question` entry in the conversation-owned `conversation.composer` keyed slot.
+Web question feature plugin: its browser half registers the `question` entry in the conversation-owned `conversation.composer` keyed slot. Its host half is empty on purpose — mounting `dsh-tool-ask-user` there put the tool in the registry's GLOBAL layer, which merges into every agent regardless of the preset that composed it, so a two-tool benchmark preset really presented three. Rendering a question is a host UI capability; having the tool is an agent capability, so the `tool-ask-user` row belongs to the presets that want it (and to the TUI composition, which has no presets).
 
 The component renders one question at a time with progress navigation, single- and multi-select choices, recommendation badges derived from label suffixes, and custom answers. A multi-select draft keeps its selected labels while the user opens or edits the custom answer, so its submitted item may carry both `selected` and `custom`; a single-select custom answer remains exclusive. Question detail reuses the assistant-output `MarkdownText` primitive, including its GFM rendering and untrusted-content policy. The capped card keeps its title, navigation, and submission actions fixed while long detail and choices share an internal scroll region. Single-select choices advance immediately, and Enter submits once every question is answered or skipped; Enter during IME composition confirms the input candidate without advancing. It submits one structured answer batch for the whole request: “Skip this question” retains other drafts and emits the existing blank `{ selected: [] }` shape for that item, while close rejects the whole wait as `ASK_CANCELLED`.