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>

tests/test_cli.py

@@ -183,3 +183,19 @@ class TestCheckUpdateRetry:
         assert result == "error"
         assert "网络超时" in captured.out
         assert "已重试 3 次" in captured.out
+
+
+class TestVersionCompare:
+    def test_newer_remote_triggers_update(self):
+        assert cli._is_newer_version("1.5.0", "1.4.2") is True
+
+    def test_equal_versions_no_update(self):
+        assert cli._is_newer_version("1.5.0", "1.5.0") is False
+
+    def test_local_ahead_of_release_no_downgrade_prompt(self):
+        """发版窗口期本地装了 main(更新)时,不能提示"有更新"诱导降级。"""
+        assert cli._is_newer_version("1.4.2", "1.5.0") is False
+
+    def test_unparseable_falls_back_to_inequality(self):
+        assert cli._is_newer_version("2026.06-beta", "1.5.0") is True
+        assert cli._is_newer_version("1.5.0", "1.5.0-dev") is True