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/apiproxy/src/api-proxy.ts

@@ -39,7 +39,7 @@ import type {
   AskUserQuestionAnswer, AskUserQuestionItem, AskUserQuestionRequest,
 } from '@deepseek-ai/dsh-user-interaction'
 import { UserInteractionError } from '@deepseek-ai/dsh-user-interaction'
-import { pickNativeDirectory } from './native-directory-picker.ts'
+import { DirectoryPickerError } from '@deepseek-ai/dsh-host-directory-picker'
 
 /** Page size when history is called without maxMessages. */
 const DEFAULT_MAX_MESSAGES = 50
@@ -193,6 +193,14 @@ async function summarizeCold(persistence: SessionPersistence, meta: SessionHeade
   }
 }
 
+/** Map a browse-primitive failure onto the wire error vocabulary (unknown throws stay internal). */
+function directoryError(error: unknown): RpcError {
+  if (error instanceof DirectoryPickerError) {
+    return { code: error.code, message: error.message, details: { path: error.path } }
+  }
+  return { code: 'internal', message: error instanceof Error ? error.message : String(error), details: {} }
+}
+
 /** Resolved Host routing and project-directory defaults consumed by the API implementation. */
 export interface ApiProxyDefaults {
   provider: string
@@ -201,8 +209,6 @@ export interface ApiProxyDefaults {
   cwd: string
   /** Parent directory for name-created workspaces. */
   workspaceRoot: string
-  /** Native single-directory picker; injectable for carrier tests. */
-  pickDirectory?: (signal: AbortSignal) => Promise<string | null>
 }
 
 /** The tool/call payload fields the presenter path reads. */
@@ -990,12 +996,21 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
           provider: defaults.provider,
           model: defaults.model,
           attachedSessions: ctx.agents.list().length,
+          directoryPicker: ctx.directoryPicker.capability().kind,
         }))
       },
 
       async pickDirectory(request, signal) {
+        const capability = ctx.directoryPicker.capability()
+        if (capability.kind !== 'dialog') {
+          return err(request, {
+            code: 'directory-picker-unavailable',
+            message: `host.pickDirectory needs the dialog capability; the composed picker serves "${capability.kind}"`,
+            details: { capability: capability.kind },
+          })
+        }
         try {
-          const path = await (defaults.pickDirectory ?? pickNativeDirectory)(signal)
+          const path = await capability.pick(signal)
           return ok(request, { path })
         } catch (error: unknown) {
           if (signal.aborted) {
@@ -1012,6 +1027,38 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
           })
         }
       },
+
+      async listDirectory(request) {
+        const capability = ctx.directoryPicker.capability()
+        if (capability.kind !== 'browse') {
+          return err(request, {
+            code: 'directory-picker-unavailable',
+            message: `host.listDirectory needs the browse capability; the composed picker serves "${capability.kind}"`,
+            details: { capability: capability.kind },
+          })
+        }
+        try {
+          return ok(request, await capability.list(request.payload.path))
+        } catch (error: unknown) {
+          return err(request, directoryError(error))
+        }
+      },
+
+      async createDirectory(request) {
+        const capability = ctx.directoryPicker.capability()
+        if (capability.kind !== 'browse') {
+          return err(request, {
+            code: 'directory-picker-unavailable',
+            message: `host.createDirectory needs the browse capability; the composed picker serves "${capability.kind}"`,
+            details: { capability: capability.kind },
+          })
+        }
+        try {
+          return ok(request, { path: await capability.createDirectory(request.payload.path, request.payload.name) })
+        } catch (error: unknown) {
+          return err(request, directoryError(error))
+        }
+      },
     },
 
     commands: {