feat(host): directory-picker capability seam with dialog and browse backends

The web GUI's folder picking was hardwired to one interaction: a native
OS chooser compiled into the gateway, unusable for remote deployments
and swappable only by editing apiproxy source.

Directory picking becomes a three-package capability seam in
packages/host: ctx.directoryPicker returns a discriminated capability —
dialog (the extracted native chooser; host-display only) or browse
(new: one-level listing + child creation over Node stdlib, hidden flags
host-stamped, symlinks followed, ancestry crumbs; remote-capable). The
gateway injects the seam, advertises the kind via
host.describe.directoryPicker, serves host.listDirectory /
host.createDirectory under browse, and answers
directory-picker-unavailable across kinds. cordis.yml is the swap
point; apps/cli keeps dialog mounted, so behavior is unchanged until
the in-app browser PR flips the default. The connection fixture serves
a deterministic browse tree; WorkspacesService gains the browse calls
the browser UI will drive. Decision record:
.agents/notes/implemented/architecture/2026-07-28-directory-picker-capability-seam.md

packages/host/README.zh.md

@@ -8,5 +8,8 @@ dsh web GUI 的宿主侧：所有客户端形态共用的 API 网关，以及承
 |---|---|---|
 | `apiproxy/` | 共享 API 网关：零 Node 依赖的 TS 协议契约（`src/api/`）、fetch 载体对（宿主侧 `toFetchHandler`、客户端侧 `AbstractApiClient`），以及基于 `ctx.agents`／`ctx.workspace` 的宿主实现 | `ctx.apiProxy` |
 | `webserver/` | 纯 HTTP 路由注册载体：激活即监听的 `node:http` 服务器；路由以命名的 `exact`／`prefix` 处理器注册 | `ctx.httpServer` |
+| `directory-picker/` | 工作区目录选择 seam：网关的 picker RPC 委托的可辨识 `dialog`／`browse` 能力 | `ctx.directoryPicker` |
+| `directory-picker-dialog/` | 原生 OS 选择器后端（osascript／PowerShell／Zenity+KDialog）；仅宿主屏幕可用 | （注册 `ctx.directoryPicker`） |
+| `directory-picker-browse/` | 应用内浏览后端：基于 Node 标准库的列举／创建原语；支持远程 | （注册 `ctx.directoryPicker`） |
 
 `apiproxy` 在设计上与传输方式无关——它不注册任何路由；载体自行包装 `ctx.apiProxy`。HTTP 载体路由（连同其 `/api` 浏览器信任栅栏）由 [`client/connection`](../client/connection/README.md) 的 node 半侧挂载，这正是该包住在 client 组的原因：它拥有这条线的两端。