docs: link the integration guide from both READMEs, fix three inaccuracies

Adds a pointer to docs/extending-codeman.md at the end of the API section in
README.md and README.zh-CN.md, so the guide is reachable from where people
read about endpoints rather than only from CLAUDE.md.

Reading the README's programmatic guide alongside the new page surfaced three
errors in it, all now fixed:

- POST /api/sessions/:id/input takes `useMux`, not `useScreen`. The latter is
  a legacy name that no longer appears in the schema.
- The page told integrators to send `\r` to submit. With `useMux: true` the
  server delivers text and Enter as two separate writes (writeViaMux does
  send-keys -l then send-keys Enter), so appending `\r` is wrong.
- "Unwrap the envelope" was incomplete: a few legacy GETs put the payload at
  the top level, so the advice is now `body.data ?? body`.

Also cross-references the README's programmatic guide, which covers the
in-session case (CODEMAN_MUX, CODEMAN_API_URL, CODEMAN_SESSION_ID,
CODEMAN_HOOK_SECRET_FILE) that the new page deliberately does not duplicate,
and documents the optional clientId/seq exactly-once fields.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.zh-CN.md

@@ -802,6 +802,8 @@ Codeman 会注册 Claude Code hook，它们 `POST /api/hook-event`（`permission
 | `POST` | `/api/clipboard`                | 把文本推送到所有已连接浏览器（`{text}`） |
 | `GET`  | `/api/sessions/:id/run-summary` | 时间线 + 统计                            |
 
+> **想在 Codeman 之上做集成？**[`docs/extending-codeman.md`](docs/extending-codeman.md)（英文）是集成指南：把你自己的界面作为标签页嵌入、订阅 SSE 事件流以便在 agent 需要你时做出响应、用脚本驱动 Codeman，以及动手前值得先了解的那些坑。Codeman 刻意不提供插件运行时，所以一个集成就是你自己的进程在讲 HTTP。
+
 ---
 
 ## 架构