feat(composer): replicate WorkBuddy branch label and context usage (TODO 21)

src/main/gateway/index.ts

@@ -10,7 +10,7 @@
  * 模型路由、上下文裁剪、工具编排由上层（agent/loop.ts）负责。
  */
 
-import type { GatewayConfig, ToolSpec } from '@shared/types'
+import type { GatewayConfig, TokenUsage, ToolSpec } from '@shared/types'
 import { resolveOpenAIEndpoint } from '@shared/openai-endpoint'
 import { parseSse } from './sse'
 import { buildResponsesRequest, parseResponsesStream } from './responses'
@@ -34,6 +34,7 @@ export type GatewayDelta =
   | { type: 'text'; text: string }
   | { type: 'reasoning'; text: string }
   | { type: 'tool_calls'; calls: WireToolCall[] }
+  | { type: 'usage'; usage: TokenUsage }
   | { type: 'finish'; reason: string | null }
 
 export interface GatewayStreamRequest {