feat(experience): one-liner install/update docs catch up + version-watching skill

The one-liner entry docs were untouched by the backend upgrade — new users
following "帮我安装 Agent Reach" would still get the old world. Now:

- docs/install.md: xiaohongshu section rewritten (desktop OpenCLI with the
  one manual extension click / server xiaohongshu-mcp with QR + 150MB
  first-run warning / legacy xhs-cli kept as fallback); Reddit section says
  plainly there is no zero-config path; optional-channel menu gains OpenCLI;
  upstream table matches the new routing with a doctor --json pointer
- docs/update.md: full rewrite — upgrade-only-what-exists tool refresh
  (incl. npm update for OpenCLI/mcporter, pinned git source for rdt-cli),
  one-time OpenCLI offer for desktops, skill sync now rides doctor's
  auto-sync instead of an inline python blob, WeChat-era content removed
- check-update / watch: "update available" now leads with the one-liner
  (帮我更新 Agent Reach：<update.md>) which updates package + tools + skill;
  bare pip stays as the package-only alternative
- fix: version compare was `!=` — a local build AHEAD of the latest release
  (installed from main during a release window) was told to "update" into a
  downgrade; now semantic tuple compare with fallback (+4 tests)
- skill (zh+en): description gains a NOT-for boundary (fetch-only, no write
  ops, defer to dedicated skills); new standing rule — after substantial
  tasks run check-update and append a one-liner update hint to the wrap-up,
  never interrupting or nagging
- SKILL_en.md restructured to the same three-layer skeleton as the Chinese
  version (124-line body, shared references/) instead of a 263-line
  everything-inline page

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

agent_reach/skill/SKILL.md

@@ -14,6 +14,9 @@ description: >
   Zero config for 6 channels. Run `agent-reach doctor --json` to see which
   backend serves each platform right now.
 
+  NOT for: 写报告/数据分析/翻译等内容加工（本 skill 只负责从互联网获取内容）；
+  发帖/评论/点赞等写操作；已有专门 skill 的平台（先用专门 skill）。
+
   【路由方式】SKILL.md 包含路由表和常用命令，复杂场景需按需阅读对应分类的 references/*.md。
   分类：search / social (小红书/推特/B站/V2EX/Reddit) / career(LinkedIn) / dev(github) / web(网页/文章/RSS) / video(YouTube/B站/播客)。
 triggers:
@@ -46,6 +49,11 @@ metadata:
 2. **声明你在用什么**：开始干活前说一句「使用 agent-reach 的 X 平台 / Y 后端」。
 3. **失败按 references 里的重试链处理**，不要瞎猜命令。
 4. **全网调研类任务**：组合多平台（Exa 搜索 + Twitter/Reddit 看讨论 + 小红书/B站看中文场景），并行收集再汇总。
+5. **替用户盯版本**：完成一次较大的调研/多平台任务后，顺手跑
+   `agent-reach check-update`（很快，一个 API 调用）。有新版就在收尾汇报里附一句：
+   「Agent Reach 有新版 vX.Y.Z，复制这句话给我即可更新：帮我更新 Agent Reach：
+   https://raw.githubusercontent.com/Panniantong/agent-reach/main/docs/update.md」。
+   不要中断当前任务去更新，也不要重复提醒同一个版本。
 
 ## 路由表