docs(i18n): cookbook batch — six bilingual pairs via the committed pipeline

六篇 cookbook 全部配对（adding-a-package / adding-a-tool /
adding-a-vendored-package / adding-an-llm-adapter /
extension-cookbook / responding-to-pr-review-on-a-stack）：译文由
进仓流水线产出（translation-prompt.md 全占位符渲染 + 5 组金标
few-shot），双侧语言切换行齐备，六篇加入 manifest required（15）。
另补 prompt 的 When translating into English 一节（此前为占位）：
标点/术语双向绑定/主语显化/惯用语概念还原/语域，与
translation-rules 的中文先行条款一致。

docs/cookbook/adding-an-llm-adapter.md

@@ -1,5 +1,7 @@
 # Cookbook: adding an LLM adapter
 
+English | [中文](adding-an-llm-adapter.zh.md)
+
 How to connect a new model provider. Reference implementations: `packages/llm/llm-deepseek` (hand-rolled HTTP/SSE) and `packages/llm/llm-pi-ai` (wrapping an LLM library). Read the `StreamChunk` doc in `packages/llm/llm/src/types.ts` first — it records the protocol conventions both adapters were verified against.
 
 ## The shape