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/tests/fetch-carrier.spec.ts

@@ -76,7 +76,7 @@ function fakeApi(overrides: Partial<{ muxFrames: MuxFrame[]; hostFrames: HostFra
     },
     host: {
       async describe(request) {
-        return { rpcId: request.rpcId, result: { ok: true, value: { version: 'v', cwd: '/w', attachedSessions: 0, directoryPicker: 'native' as const } } }
+        return { rpcId: request.rpcId, result: { ok: true, value: { version: 'v', cwd: '/w', attachedSessions: 0 } } }
       },
       async pickDirectory(request) {
         return { rpcId: request.rpcId, result: { ok: true, value: { path: null } } }