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.

agent_reach/skill/SKILL.md

@@ -255,7 +255,7 @@ print(result[0]["error"])  # 提示使用站内搜索或 Exa channel
 
 > No auth required. Results are public JSON. V2EX 节点名见 https://www.v2ex.com/planes
 
-## 雪球 / Xueqiu (public API)
+## 雪球 / Xueqiu (需要登录 Cookie)
 
 ```python
 from agent_reach.channels.xueqiu import XueqiuChannel
@@ -274,7 +274,7 @@ stocks = ch.search_stock("茅台", limit=5)
 for s in stocks:
     print(f"{s['name']} ({s['symbol']}) - {s['exchange']}")
 
-# 热门帖子
+# 热门帖子（v4 timeline，含作者、点赞数、正文）
 # 返回字段：id, title, text(前200字), author, likes, url
 posts = ch.get_hot_posts(limit=10)
 for p in posts:
@@ -287,7 +287,7 @@ for s in hot:
     print(f"#{s['rank']} {s['name']} ({s['symbol']}): {s['current']} ({s['percent']}%)")
 ```
 
-> 无需登录。自动获取会话 Cookie，所有公开 API 均可直接使用。
+> 需要登录 Cookie。先在 Chrome 登录 xueqiu.com，再运行 `agent-reach configure --from-browser chrome`。
 
 ## RSS (feedparser)