refactor(client): command decorations replace the hostBacked contribution mode

A popup on a host command is not a second command — it is what that
command's BARE invocation does on this client. CommandContribution loses
hostBacked (contributions are pure client commands again; a host-name
collision fails loud, unchanged for /model), and the contract gains
CommandDecoration + command.decorate(): key = the HOST command name, no
catalog row, no claim participation. Dispatch consults decorations only on
the bare paths (menu pick / bare enter) after the host row resolves; space
and argued enter never see them — the two edges hostBacked had to guard
explicitly hold by construction in the decoration model. A decorated name
with no host row in the session's directory never fires (a decoration
cannot manufacture a command).

ui-permission switches register→decorate with zero behavior change
(options still read the permissions projection; a pick still submits
'/permission <preset>'). Specs rewrite to the decoration semantics: no
catalog row, bare-enter popup vs argued-enter host claim, space host
claim, no-host-row miss, unavailable fall-through, duplicate fail-loud.

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

@@ -4,7 +4,7 @@
 
 客户端命令业务面（`ctx.command`）：以会话为 key 的命令目录缓存、带 matchSpace／matchEnter 裁决钩子的 `/` 命令 source、三型派发（execute／popupSelect／leadingInput），以及面向业务包的 popupSelect 注册面。契约：[Web 命令业务面 Agent Note（agent 决策记录）](../../../.agents/notes/implemented/architecture/2026-07-25-web-command-surfaces-and-assembly.zh.md)。
 
-`src/client/contract.ts` 是冻结的业务表层：`CommandServiceContract.register(name, spec)` 是业务包消费的全部内容；`CommandUiSpec{options, onSelect}` 让 popup 数据自给自足——壳组件归本包所有，业务永远见不到它。命令三型按每次派发派生，绝不在注册时定型：带 `input` 的 host descriptor 是 leadingInput，注册了 `CommandUiSpec` 的是 popupSelect，其余全部是 execute。
+`src/client/contract.ts` 是冻结的业务表层：`CommandServiceContract.register(name, spec)` 与 `decorate(name, spec)` 是业务包消费的全部内容；`CommandUiSpec{options, onSelect}` 让 popup 数据自给自足——壳组件归本包所有，业务永远见不到它。contribution 是 client 自有命令（与 host 同名碰撞即 fail-loud）；decoration（装饰）则把裸调用 popup 挂在**已存在的** host 命令上——host 保留目录行、带参 claim（space / 带参 enter）与生命周期记账，被装饰的名字若在会话目录中无 host 行则装饰永不触发。命令三型按每次派发派生，绝不在注册时定型：带 `input` 的 host descriptor 是 leadingInput，注册了 `CommandUiSpec` 的是 popupSelect，其余全部是 execute。
 
 `CommandDirectory`（`src/client/directory.ts`）是唯一的 wire 派生缓存，以会话为 key：每个会话恒为 agent-backed，因此 `command.list({sessionId})` 是唯一的寻址形状，source 的 scope 出生 `warm` 钩子会预热该会话的缓存项。缓存项由 `commands/changed` 类型化事件软失效（重拉在途期间旧快照继续服务），由 `connection/reset` 硬失效，并以 epoch 把关，被取代的旧拉取永远无法覆盖更新的结果。`matchSpace` 只凭该缓存同步应答；`matchEnter` 在 SubmitAttempt 信号上强等缓存，预热失败即拒绝——`/` 开头的一行绝不会被静默降级为普通提示词。