fix: 恢复 ACP usage 传递（usage_update + PromptResponse.usage）

撤销审计 §4.1 的原修复（删除 usage_update 以求严格 v1 stable 合规）。
现实中的所有主流 ACP 客户端（Zed、Cursor 等）实现的是 unstable spec，
删除 usage_update 后客户端 context 使用量一律显示 0/0，严重破坏 UX。

SDK 已包含 UsageUpdate 类型（sessionUpdate: 'usage_update'，字段 used + size
+ 可选 cost）和 PromptResponse.usage 根字段（UNSTABLE 但被广泛实现），这是
context 使用量报告的唯一标准化载体，故选择优先保证 interop。

变更：
- bridge/forwarding.ts: 收到 'result' 消息且 lastAssistantTotalUsage !== null
  时发送 usage_update
  - used = 最近一条 assistant 消息的 input + output + cache_read + cache_creation
    token 总和（≈ 当前上下文占用）
  - size = lastContextWindowSize（默认 200000，通过 modelUsage prefix-match 解析）
  - compact_boundary 时不发（不知道压缩后实际占用；下一轮 result 会自然修正）
- agent/promptFlow.ts: PromptResponse 根部添加 usage 字段，并镜像到
  _meta.claudeCode.usage 供消费者任选读取路径
- 测试更新：bridge.test.ts 三个相关 test 改为断言 usage_update 被发出且
  used/size 正确；agent.test.ts 改为断言 root usage 存在
- 审计文档 §4.1 标记为已撤销，添加决策回滚说明

验证：bun run precheck 全通过（typecheck + lint + 5851 tests）
ACP service tests: 176 pass / 0 fail

Co-Authored-By: glm-5.2 <zai-org@claude-code-best.win>

src/services/acp/agent/promptFlow.ts

@@ -109,31 +109,34 @@ async function prompt(
     // channel. The title is derived from the first user prompt.
     await emitSessionInfoUpdate(this, params.sessionId, promptInput)
 
-    // Per extensibility.mdx:39 the root of PromptResponse is reserved —
-    // stable v1 defines only `stopReason` (+ optional `_meta`). Token usage
-    // is therefore carried under the `_meta.claudeCode.usage` extension
-    // namespace rather than as a non-spec root field. thoughtTokens are
-    // included in totalTokens so reported totals match billable tokens;
-    // until bridge.ts tracks them they are reported as 0.
+    // Per session-usage.mdx RFD and the bundled SDK schema, PromptResponse
+    // carries an optional `usage` field at the root with cumulative token
+    // totals for the session. The field is UNSTABLE in v1 but is implemented
+    // by all major ACP clients. We additionally mirror the same payload into
+    // `_meta.claudeCode.usage` for consumers that read the vendor namespace.
+    // thoughtTokens are reported as 0 until the bridge tracks them, but are
+    // included in totalTokens so totals match the sum of components.
     if (usage) {
       const thoughtTokens = 0
+      const usagePayload = {
+        inputTokens: usage.inputTokens,
+        outputTokens: usage.outputTokens,
+        cachedReadTokens: usage.cachedReadTokens,
+        cachedWriteTokens: usage.cachedWriteTokens,
+        thoughtTokens,
+        totalTokens:
+          usage.inputTokens +
+          usage.outputTokens +
+          usage.cachedReadTokens +
+          usage.cachedWriteTokens +
+          thoughtTokens,
+      }
       return {
         stopReason,
+        usage: usagePayload,
         _meta: {
           claudeCode: {
-            usage: {
-              inputTokens: usage.inputTokens,
-              outputTokens: usage.outputTokens,
-              cachedReadTokens: usage.cachedReadTokens,
-              cachedWriteTokens: usage.cachedWriteTokens,
-              thoughtTokens,
-              totalTokens:
-                usage.inputTokens +
-                usage.outputTokens +
-                usage.cachedReadTokens +
-                usage.cachedWriteTokens +
-                thoughtTokens,
-            },
+            usage: usagePayload,
           },
         },
       }