feat(host): adaptive directory-picker default via -auto chooser

Add @deepseek-ai/dsh-host-directory-picker-auto, a node-half-only chooser
that samples the host situation once at boot (bind host via a new
httpServer.host getter, SSH markers, platform, DISPLAY/WAYLAND_DISPLAY)
and mounts the matching dual-face backend (-native or -browse) as a real
Loader entry in the in-memory root tree; the effect disposer removes it.
Entry-level mounting keeps the seam's one-row-swaps-both-faces invariant:
the client module table discovers the mounted backend's browser half
exactly as a config row's. apps/cli now composes -auto as its
directory-picker row; composing a backend row directly remains the pin.

packages/host/directory-picker-auto/src/index.ts

@@ -0,0 +1,52 @@
+/**
+ * Adaptive chooser of the directory-picker seam: resolves the host's
+ * situation once at boot (bind host, SSH launch, display session) and mounts
+ * the matching dual-face backend — `-native` or `-browse` — as a real Loader
+ * entry in the in-memory root tree. Because the backend arrives as an
+ * ordinary entry, its browser half is discovered exactly as a config-row's
+ * would be, so the seam's one-row-swaps-both-faces invariant holds for the
+ * resolved choice; pinning an interaction remains composing that backend row
+ * directly instead of this one.
+ * @module @deepseek-ai/dsh-host-directory-picker-auto
+ */
+
+import type { Context } from 'cordis'
+// Empty type imports carry the `loader` and `httpServer` Context merges for the reads below.
+import type {} from '@cordisjs/plugin-loader'
+import type {} from '@deepseek-ai/dsh-host-webserver'
+import type { DirectoryPickerBackendKind } from './resolve.ts'
+import { resolveDirectoryPickerBackend } from './resolve.ts'
+
+export type { DirectoryPickerBackendKind, DirectoryPickerEnv, DirectoryPickerHostFacts } from './resolve.ts'
+export { resolveDirectoryPickerBackend } from './resolve.ts'
+
+/** Cordis plugin name. */
+export const name = 'directory-picker-auto'
+/** Required services: the effective bind host (`httpServer`) and the entry tree the backend mounts into (`loader`). */
+export const inject = ['httpServer', 'loader']
+
+/** Backend package per resolved kind — fixed composition vocabulary, not a tunable. */
+const BACKEND_PACKAGES: Record<DirectoryPickerBackendKind, string> = {
+  native: '@deepseek-ai/dsh-host-directory-picker-native',
+  browse: '@deepseek-ai/dsh-host-directory-picker-browse',
+}
+
+/**
+ * Resolve the backend from one boot-time sample and mount it as a Loader
+ * entry; the effect's disposer removes the entry, so unloading this plugin
+ * unloads both faces of the mounted backend with it.
+ * @param ctx - cordis context carrying the injected `httpServer` and `loader`.
+ */
+export async function apply(ctx: Context): Promise<void> {
+  const backend = resolveDirectoryPickerBackend({
+    bindHost: ctx.httpServer.host,
+    platform: process.platform,
+    env: process.env,
+  })
+  await ctx.effect(async () => {
+    // Root-tree create: the Loader root is in-memory (write() is a no-op), so
+    // the mounted row can never be persisted back into a config file.
+    const id = await ctx.loader.create({ name: BACKEND_PACKAGES[backend] })
+    return () => { ctx.loader.remove(id) }
+  }, 'directory-picker-auto: backend entry')
+}