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 {

src/main/gateway/responses.ts

@@ -149,6 +149,22 @@ export async function* parseResponsesStream(stream: ReadableStream<Uint8Array>,
         const response = object(payload.response)
         if (response.error || response.status !== 'completed' || !Array.isArray(response.output)) throw error('Responses 上游未返回合法成功终态')
         for (let i = 0; i < response.output.length; i++) yield* output({ output_index: i }, object(response.output[i]), true)
+        const usage = object(response.usage)
+        const inputTokens = usage.input_tokens
+        const outputTokens = usage.output_tokens
+        const totalTokens = usage.total_tokens
+        if (typeof inputTokens === 'number' && typeof outputTokens === 'number' && typeof totalTokens === 'number') {
+          const cachedTokens = object(usage.input_tokens_details).cached_tokens
+          yield {
+            type: 'usage',
+            usage: {
+              inputTokens,
+              outputTokens,
+              totalTokens,
+              ...(typeof cachedTokens === 'number' ? { cachedTokens } : {})
+            }
+          }
+        }
         completed = true
         break
       }