feat(host,client): compose directory picking through slots — dual-face -native, no wire advertisement

ui-workspace's two trigger surfaces each declare a single-kind directory-flow
hole (conversation.hero.workspace.directoryFlow / sidebar.workspaces.directoryFlow,
same owner contract) and keep only the trigger and the adoption: the Open-local-
folder entry renders while the surface's hole is occupied, and the occupant
reports one picked path per open through the hole's owner conversation
(open/busy/onPicked/onCancel/onError).

directory-picker-native becomes dual-face: its browser half fills both holes
with a renderless occupant driving host.pickDirectory, so the cordis.yml row
that mounts the backend also composes the client interaction — a mismatch is
impossible and a second flow package fails at client load.

With composition wiring both sides, the host.describe.directoryPicker
advertisement and the client's kind branching lose their last consumer:
the field, WorkspacesService.directoryPickerKind(), the DirectoryPickerKind
wire type, and the picker's per-open describe read are deleted. The connection
fixture now serves a deterministic pickDirectory path so the keyless snapshot
drives the full pick-then-adopt flow. ui-workspace's hand-rolled declaration
deferral is replaced by the deferRegistration helper it duplicated.

packages/host/apiproxy/src/api/host.ts

@@ -5,18 +5,6 @@
 
 import type { RpcRequest, RpcResponse } from './rpc.ts'
 
-/**
- * The composed directory-picker interaction the host serves (mirror of the
- * `ctx.directoryPicker` capability kind): `native` = one OS chooser on
- * the host display (`host.pickDirectory`); `browse` = in-app listing/creation
- * primitives (`host.listDirectory`/`host.createDirectory`). Calling a method
- * outside the advertised kind fails with `directory-picker-unavailable`.
- * The wire preserves kinds beyond the two with methods here (a merge-added
- * capability advertises before its RPCs exist); the client's documented
- * default for a kind it does not recognize is to hide the picking affordance.
- */
-export type DirectoryPickerKind = 'native' | 'browse' | (string & {})
-
 /** One directory row of a listing: a child entry or a breadcrumb ancestor. */
 export interface DirectoryEntry {
   /** Base name shown in a browser row (a root crumb carries its full path). */
@@ -51,7 +39,6 @@ export interface HostApi {
    * applied when a new agent doesn't specify them explicitly, absent when the host configures
    * no explicit default (the adapter falls back internally);
    * attachedSessions = count of currently attached sessions (those with a live agent);
-   * directoryPicker = the composed picker interaction the client renders for.
    */
   describe(request: RpcRequest<{}>): Promise<RpcResponse<{
     version: string
@@ -59,7 +46,6 @@ export interface HostApi {
     provider?: string
     model?: string
     attachedSessions: number
-    directoryPicker: DirectoryPickerKind
   }>>
 
   /**