refactor(client): carve outward interfaces for the session, workspace, layout, slash, and conversation services

Feature packages now reach these domains through interface types only:
ISession/SessionFace (identity + prompt/cancel/loadOlder + the useSession
snapshot source), ISessions, IWorkspaces, ILayout, IConversation, and the
existing SlashServiceContract now actually mounted on Context.slash. The
concrete services implement their face; wire-pump and assembly entry
points stay on the classes. The provide-channel materialization and
current-projection logic moves into SessionProvideChannel so the
production service and the client test runtime share one implementation.
The workspaces service consumes sessions through the narrow SessionsPort.

packages/client/ui-slash/src/client/index.ts

@@ -24,7 +24,8 @@ export type { SlashServiceContract } from './contract.ts'
 
 declare module 'cordis' {
   interface Context {
-    slash: SlashService
+    /** The outward face only; the concrete service stays inside this plugin. */
+    slash: import('./contract.ts').SlashServiceContract
   }
 }
 

packages/client/ui-slash/src/client/service.ts

@@ -7,7 +7,7 @@
  */
 import { Service } from 'cordis'
 import type { Context } from 'cordis'
-import type { ClientContext, SessionId, SessionsService } from '@deepseek-ai/dsh-client-runtime/client'
+import type { ClientContext, ISessions, SessionId } from '@deepseek-ai/dsh-client-runtime/client'
 import type { SlashSource } from '../types.ts'
 import { SlashController } from './controller.ts'
 import type { SlashServiceContract } from './contract.ts'
@@ -99,7 +99,7 @@ export class SlashService extends Service implements SlashServiceContract {
     return controller
   }
 
-  private sessions(): SessionsService {
+  private sessions(): ISessions {
     const sessions = this.ctx.get('sessions')
     if (sessions === undefined) throw new Error('ui-slash: sessions service unavailable')
     return sessions