fix(web-plugin-config): address review — one options snapshot per search, no public value exports

Three findings from review survived against the staged-save head:

The search provider read its options thunk per property, so a settings write
landing inside credential resolution sent the key resolved from the old
section to the endpoint named by the new one. Each operation now snapshots
once at its entry and threads that snapshot into credential resolution; a
regression test drives a commit into the middle of a search and pins that the
endpoint, model, and key all come from the section the search started on.

The /client entry exported components, controllers, and namespace constants
with no consumer, which the client export discipline allows only with sign-off.
Only types remain. The duplicate per-card Injected/Face interface pairs are
one declaration each now, so a member added to one side cannot silently miss
the other.

The credential state carries the reference it describes and its writability: a
reference change no longer projects the old answer onto the new name, an
out-of-order response for a stale reference is dropped, and a key that a
deployment sources from the process environment disables the control instead
of inviting a write the Host must refuse.

Also corrected three prose claims against the code they describe: the card's
fields do not differ by platform (the served schema does), the section's empty
line counts registered rather than visible cards and is read once, and the
search README overstated what a configuration surface learns about a key.

packages/client/ui-plugin-config/README.zh.md

@@ -34,4 +34,5 @@
 
 - **只有宿主平面的插件会出现**——由 agent preset 挂载的插件把配置内联在该 preset 的 `agent.cordis.yml` 中，且根本无法注册 settings 命名空间（同一 preset 挂载第二个会话时会因重复注册而失败），因此本分区不会列出它。编辑那些值仍是 preset 编辑器的职责。
 - **暴露是 Host 的白名单，而非插件的声明**——不在 api-proxy 白名单中的命名空间，即便其拥有方已注册，也只会得到 `settings-not-exposed`，因此在本仓库之外分发的插件无法在不改动 `packages/host/apiproxy` 的前提下让自己的配置出现在这里。
-- **shell 卡片跟随被组装的执行器**——POSIX 与 PowerShell 两个执行器家族共用 `bash` 命名空间，因为一个宿主只组装其中之一，所以该卡片的字段随平台不同，而两者都不组装的部署不会显示这张卡片。
+- **shell 卡片跟随被组装的执行器**——POSIX 与 PowerShell 两个执行器家族共用 `bash` 命名空间，因为一个宿主只组装其中之一，所以被服务的 schema 随平台不同（PowerShell 多出 `pwshPath`），尽管卡片在两者下编辑的都是同样两个字段；而两者都不组装的部署不会显示这张卡片。
+- **空态数的是已注册卡片，不是可见卡片**——命名空间未被本部署暴露的卡片什么都不渲染，但仍计入数量，因此一个都不暴露的部署看到的是空列表而非那行空态文案。该计数还只读取一次，因为渲染器会缓存根级 entry 的 inject face；之后注册的卡片不会让它变大。