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/client/ui-workspace/src/client/WorkspaceBrowser.tsx

@@ -253,8 +253,8 @@ export function WorkspaceBrowser({
   deleteWorkspace,
   insertSessionBefore,
   createWorkspace,
-  pickDirectory,
-  directoryPickerKind,
+  hasDirectoryFlow,
+  renderSlot,
 }: WorkspaceBrowserProps) {
   const workspaces = useWorkspaces(state => state.items)
   const groupBy = useStore(s => s.groupBy)
@@ -372,8 +372,8 @@ export function WorkspaceBrowser({
           anchorRef={wsPlusRef}
           useWorkspaces={useWorkspaces}
           createWorkspace={createWorkspace}
-          pickDirectory={pickDirectory}
-          directoryPickerKind={directoryPickerKind}
+          hasDirectoryFlow={hasDirectoryFlow}
+          renderDirectoryFlow={owner => renderSlot('sidebar.workspaces.directoryFlow', owner)}
           createOnly
           side="right"
           onPick={(workspaceId) => {