fix(web): keep the plan card to decisions it can actually answer

Review follow-ups on the presentation intent.

The card claimed any single-question request declaring the intent, then sent one
of two labels — so a producer offering a third option, or a multi-select batch,
lost answers the generic flow would have shown. That contradicts the intent's
own contract, so `planReviewOf` now claims only a binary single choice and
leaves everything else to the flow that can express it.

`ask()` also rejects a plan-review intent on a question with no `detail`: the
intent declares detail IS the plan, and without one a honouring UI asks the
user to approve something invisible. The client keeps its own fallback — it sits
downstream of a wire boundary — but the misconfiguration now fails at the asker.

`planReviewOf` stops being a value export of the client contract face (client
export discipline: pure helpers stay internal; the tests already import it
relatively), and the ui-question README fallback list, both languages, now
states every condition the code enforces.

packages/ui/user-interaction/README.zh.md

@@ -24,7 +24,7 @@
 
 ### 呈现意图
 
-`intent` 声明某个问题本身就是一次已知形状的决定，因此认识该标签的 UI 可以照此呈现 —— `plan-review` 表示 `detail` 是一份待审阅的计划，`dsh-plan-mode` 会在 `exit_plan_mode` 的问题上设置它。意图只塑造呈现：遵循它的 UI 回答的仍是通用 UI 会发送的那些选项标签，不认识该标签的 UI 渲染通用选项列表，因此调用方两种情况下读到的都是同一种回答形态。`approve` 指名表示批准的标签，而不依赖选项顺序；由于没有类型能把两者绑定起来，`ask()` 会以 `BAD_INTENT` 拒绝 `approve` 未命中该问题自身任一选项的意图。
+`intent` 声明某个问题本身就是一次已知形状的决定，因此认识该标签的 UI 可以照此呈现 —— `plan-review` 表示 `detail` 是一份待审阅的计划，`dsh-plan-mode` 会在 `exit_plan_mode` 的问题上设置它。意图只塑造呈现：遵循它的 UI 回答的仍是通用 UI 会发送的那些选项标签，不认识该标签的 UI 渲染通用选项列表，因此调用方两种情况下读到的都是同一种回答形态。`approve` 指名表示批准的标签，而不依赖选项顺序。有两项断言是任何类型都承载不了的，`ask()` 会以 `BAD_INTENT` 拒绝它们：`approve` 未命中该问题自身的任一选项，以及意图落在没有 `detail` 的问题上 —— 而 `detail` 正是它自称在审阅的东西。
 
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