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.

scripts/gen-doc-graphs.ts

@@ -423,7 +423,7 @@ const SERVICE_ROLES: ServiceRole[] = [
     mode: 'seam',
     implementations: ['directory-picker-native', 'directory-picker-browse'],
     consumers: ['apiproxy'],
-    note: 'Discriminated interaction capability: the native backend opens one OS chooser on the host display, the browse backend serves listing/creation primitives for the in-app browser; the gateway advertises the kind via host.describe.',
+    note: 'Discriminated interaction capability: the native backend opens one OS chooser on the host display, the browse backend serves listing/creation primitives for the in-app browser; each backend is dual-face, its browser half filling ui-workspace directory-flow slots (no wire advertisement).',
   },
   {
     key: 'httpServer',