refactor(picker): split the directory-picker faces into their own packages

The browse and native backends were dual-face packages: a Node backend plus a
browser surface under one tsconfig that referenced Client packages. That put
Client projects — and through them the Client runtime — inside the Host
compiler aggregate, which builds before the generated Remote contributions
exist. Each browser half moves to its own Client package, and both backends
become Node-only.

The interaction is still one choice: the adaptive chooser mounts the backend
and its surface as a pair of Loader entries and tears both down in reverse, so
a resolved kind still swaps both faces. Compositions that pin an interaction
directly now pin the pair, and the chooser's runtime-string package list keeps
naming everything a composing app must resolve.

packages/client/ui-directory-picker-native/src/index.ts

@@ -0,0 +1,10 @@
+/**
+ * Native directory-picker surface, node half. Pure UI plugin: the empty apply
+ * exists so the plugin appears in the host cordis.yml / Loader; the browser
+ * half ships via exports["./client"], discovered through the package.json
+ * dsh.client declaration. The OS chooser it drives lives in
+ * `@deepseek-ai/dsh-host-directory-picker-native`.
+ */
+
+/** Host plugin body — no host-side behavior for this surface plugin. */
+export function apply(): void {}