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/bridge/forwarding.ts

@@ -92,12 +92,10 @@ export async function forwardSessionUpdates(
           const subtype = msg.subtype
 
           if (subtype === 'compact_boundary') {
-            // Reset assistant usage tracking after compaction
+            // Reset assistant usage tracking after compaction. We don't emit a
+            // usage_update here because we don't know the post-compaction context
+            // size — the next prompt's result will carry the corrected value.
             lastAssistantTotalUsage = 0
-            // NOTE: usage_update is an UNSTABLE SessionUpdate discriminator (not in
-            // stable v1 schema). Token/cost info has no v1-stable carrier; we drop
-            // it from session/update and rely on PromptResponse._meta for clients
-            // that need it (see audit §4.1).
             await conn.sessionUpdate({
               sessionId,
               update: {
@@ -132,10 +130,23 @@ export async function forwardSessionUpdates(
             }
           }
 
-          // NOTE: usage_update was removed — it is an UNSTABLE SessionUpdate
-          // discriminator not present in the stable v1 schema (audit §4.1). Token
-          // and cost information is returned via PromptResponse._meta.claudeCode.usage
-          // instead.
+          // Per session-usage.mdx RFD: emit usage_update so clients can display
+          // context window utilization (e.g. "53K / 200K"). Although usage_update
+          // is currently UNSTABLE in the v1 schema, it is the only standardized
+          // carrier for context-window state and is implemented by all major ACP
+          // clients (Zed, Cursor, etc.). Strict v1-stable compliance broke this
+          // UX (clients showed 0/0), so we emit it whenever we have usage data.
+          // See audit §4.1 for the prior strict-compliance rationale and revert.
+          if (lastAssistantTotalUsage !== null) {
+            await conn.sessionUpdate({
+              sessionId,
+              update: {
+                sessionUpdate: 'usage_update',
+                used: lastAssistantTotalUsage,
+                size: lastContextWindowSize,
+              },
+            })
+          }
 
           // Determine stop reason
           const subtype = msg.subtype
@@ -307,9 +318,9 @@ export async function forwardSessionUpdates(
 
         // ── Compact boundary ───────────────────────────────────────
         case 'compact_boundary': {
+          // Don't emit usage_update here — we don't know the post-compaction
+          // context size. The next prompt's result will carry the corrected value.
           lastAssistantTotalUsage = 0
-          // NOTE: usage_update removed — UNSTABLE discriminator, not in v1 stable
-          // schema (audit §4.1). Token info flows through PromptResponse._meta.
           await conn.sessionUpdate({
             sessionId,
             update: {