feat(settings): serve every registered namespace and key plugin cards on it

A plugin that registered a settings namespace could not reach the browser
configuration page: the api-proxy filtered every read and gated every
write through two hardcoded namespace lists, and the plugin configuration
section rendered an unordered list of cards carrying an opaque id rather
than the namespace they edit. Both gates lived in this repository, so a
user-authored plugin was configurable only by hand-editing settings.yaml.

The proxy now serves whatever ctx.settings.describe() returns and adds no
boundary of its own; a name no registration answers folds into the seam's
own settings-rejected, and the settings-not-exposed code retires. The
settings seam is untouched: which client may read a namespace, and which
page renders it, are facts about consumers.

settings.plugin.item becomes a keyed slot whose key is the namespace a
card edits, following tool.call.toolview. The section reads describe once
and dispatches the intersection of the slot ledger and the served set, so
a namespace another surface owns renders nothing without declaring
anything, and a card for an uncomposed plugin is never dispatched.

docs/architecture.zh.md

@@ -197,4 +197,4 @@ idle inject:
 | fork 活跃会话 | 调用 `ctx.sessions.fork(source, boundary?, childSessionId?)` |
 | 将注册项限定到单个 agent | 使用其 `agent.ctx`（参见 Agent 作用域） |
 
-[扩展实操手册](cookbook/extension-cookbook.md)将功能映射到能力；指南涵盖[包](cookbook/adding-a-package.md)、[工具](cookbook/adding-a-tool.md)、[LLM 适配器](cookbook/adding-an-llm-adapter.md)、[Chat 节点](cookbook/adding-a-conversation-node.md)和 [vendored 包](cookbook/adding-a-vendored-package.md)。
+[扩展实操手册](cookbook/extension-cookbook.md)将功能映射到能力；指南涵盖[包](cookbook/adding-a-package.md)、[工具](cookbook/adding-a-tool.md)、[LLM 适配器](cookbook/adding-an-llm-adapter.md)、[Chat 节点](cookbook/adding-a-conversation-node.md)、[设置卡片](cookbook/adding-a-settings-card.md)和 [vendored 包](cookbook/adding-a-vendored-package.md)。