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/client/runtime/tests/fake-api.ts

@@ -80,11 +80,22 @@ export class FakeApiClient implements IApiClient {
     payload => Promise.resolve(ok({ selected: { provider: payload.provider, model: payload.model } }))
   onPrompt: (payload: unknown) => Promise<RpcResponse<{ accepted: true }>> = () => Promise.resolve(ok({ accepted: true as const }))
   onCancel: (payload: unknown) => Promise<RpcResponse<{ accepted: true }>> = () => Promise.resolve(ok({ accepted: true as const }))
-  onDescribe: (payload: unknown) => Promise<RpcResponse<{ version: string; cwd: string; attachedSessions: number }>> =
-    () => Promise.resolve(ok({ version: '0-fake', cwd: '/f', attachedSessions: 0 }))
+  onDescribe: (payload: unknown) => Promise<RpcResponse<{ version: string; cwd: string; attachedSessions: number; directoryPicker: 'dialog' | 'browse' }>> =
+    () => Promise.resolve(ok({ version: '0-fake', cwd: '/f', attachedSessions: 0, directoryPicker: 'browse' as const }))
   onPickDirectory: (payload: unknown) => Promise<RpcResponse<{ path: string | null }>> =
     () => Promise.resolve(ok({ path: null }))
 
+  onListDirectory: (payload: unknown) => Promise<RpcResponse<{
+    path: string
+    home: string
+    crumbs: { name: string; path: string; hidden: boolean }[]
+    entries: { name: string; path: string; hidden: boolean }[]
+  }>> =
+    () => Promise.resolve(ok({ path: '/home/fake', home: '/home/fake', crumbs: [{ name: '/', path: '/', hidden: false }], entries: [] }))
+
+  onCreateDirectory: (payload: unknown) => Promise<RpcResponse<{ path: string }>> =
+    () => Promise.resolve(ok({ path: '/home/fake/new' }))
+
   private readonly muxConns: StreamConn<MuxFrame>[] = []
   private readonly hostConns: StreamConn<HostFrame>[] = []
 
@@ -106,6 +117,8 @@ export class FakeApiClient implements IApiClient {
   readonly host: IApiClient['host'] = {
     describe: (payload: unknown) => this.record('host.describe', payload, this.onDescribe(payload)),
     pickDirectory: (payload: unknown) => this.record('host.pickDirectory', payload, this.onPickDirectory(payload)),
+    listDirectory: (payload: unknown) => this.record('host.listDirectory', payload, this.onListDirectory(payload)),
+    createDirectory: (payload: unknown) => this.record('host.createDirectory', payload, this.onCreateDirectory(payload)),
   }
 
   onWorkspaceList: (payload: unknown) => Promise<RpcResponse<{ items: never[] }>> = () => Promise.resolve(ok({ items: [] }))