feat: 流式展示设计 Agent 对话

需求：设计 Agent 对话与确认生成的回复需要实时展示。

实现：统一通过 Agent Gateway 提交 Turn，接收并去重 assistant delta，以 canonical Workspace 收口，并修复跨项目旧请求回写竞态。

shared/image-workspace.ts

@@ -98,12 +98,24 @@ export type DesignGenerationTasksSnapshotEvent = {
   type: 'design.generation_tasks.snapshot';
   workspaceId: string;
   workspaceViewRevision: number;
+  workspace: DesignWorkspace;
   generationTasks: DesignGenerationTask[];
 };
 
+export type DesignAssistantDeltaEvent = {
+  id: string;
+  type: 'design.assistant.delta';
+  workspaceId: string;
+  clientTurnId: string;
+  turnRevision: number;
+  chunkIndex: number;
+  delta: string;
+};
+
 export type DesignWorkspaceEvent =
   | DesignGenerationTaskUpdatedEvent
-  | DesignGenerationTasksSnapshotEvent;
+  | DesignGenerationTasksSnapshotEvent
+  | DesignAssistantDeltaEvent;
 
 export type DesignWorkspaceBootstrap = {
   capabilities: DesignCapabilities;