fix(xueqiu): fix 400 errors by using browser cookies and correct headers

The Xueqiu stock API requires a login session token (xq_a_token) that
is generated by Xueqiu's frontend JavaScript and cannot be obtained by
simply visiting the homepage. This caused persistent HTTP 400 (error
code 400016) for all users.

Changes:
- _ensure_cookies(): add three-level priority — config file (saved by
  --from-browser) → live Chrome cookies via browser_cookie3 → homepage
  fallback. The homepage-only approach only ever got acw_tc (anti-DDoS
  token), never xq_a_token.
- _get_json(): switch User-Agent from "agent-reach/1.0" to a real Chrome
  UA, and add Referer: https://xueqiu.com/ to all API requests.
- get_hot_posts(): replace the defunct /statuses/hot/listV3.json endpoint
  (returns empty body) with the v4 public timeline endpoint; correctly
  parse item.data as a JSON string to extract author, text, and likes.
- cookie_extract.py: add Xueqiu to PLATFORM_SPECS and configure_from_browser
  so that `agent-reach configure --from-browser chrome` now also saves
  Xueqiu cookies (only when xq_a_token is present).
- check(): improve error message to direct users to --from-browser instead
  of suggesting a proxy.
- Fix urllib.parse.quote usage (was using urllib.request.quote).
- Update tier and backends description to reflect cookie requirement.
- Add 2 new tests: cookie loading from config, Referer/UA header verification.
- Update docs: README, install guide, troubleshooting, SKILL.md, CHANGELOG.

README.md

@@ -80,7 +80,7 @@ AI Agent 已经能帮你写代码、改文档、管项目——但你让它去
 | 💬 **微信公众号** | 搜索 + 阅读公众号文章（全文 Markdown） | — | 无需配置 |
 | 📰 **微博** | 热搜、搜索内容/用户/话题、用户动态、评论 | — | 无需配置 |
 | 💻 **V2EX** | 热门帖子、节点帖子、帖子详情+回复、用户信息 | — | 无需配置 |
-| 📈 **雪球** | 股票行情、搜索股票、热门帖子、热门股票排行 | — | 无需配置 |
+| 📈 **雪球** | 股票行情、搜索股票、热门帖子、热门股票排行 | — | 告诉 Agent「帮我配雪球」 |
 | 🎙️ **小宇宙播客** | — | 播客音频转文字（Whisper 转录，免费 Key） | 告诉 Agent「帮我配小宇宙播客」 |
 
 > **不知道怎么配？不用查文档。** 直接告诉 Agent「帮我配 XXX」，它知道需要什么、会一步一步引导你。