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.

tsconfig.host.json

@@ -33,6 +33,7 @@
   ],
   "exclude": [
     "packages/client/**",
+    "packages/host/directory-picker-native/**",
     "scripts/client-bundle-purity.spec.ts"
   ],
   "references": [
@@ -168,7 +169,6 @@
     { "path": "./packages/host/apiproxy" },
     { "path": "./packages/host/directory-picker" },
     { "path": "./packages/host/directory-picker-browse" },
-    { "path": "./packages/host/directory-picker-native" },
     { "path": "./packages/host/webserver" },
     { "path": "./packages/sdk/sdk-client" },
     { "path": "./packages/sdk/helper" },