feat: add Xueqiu (雪球) channel (#198)

* feat: add Xueqiu (雪球) channel for stock quotes and community posts

Add a Tier 0 (zero-config) channel for Xueqiu, China's popular stock
market and investment community platform. Uses auto-generated session
cookies via http.cookiejar — no login required.

Supported methods:
- get_stock_quote(symbol) — real-time quotes (A/HK/US markets)
- search_stock(query) — search by name or code
- get_hot_posts(limit) — trending community posts
- get_hot_stocks(limit, stock_type) — popular stocks leaderboard

Inspired by https://github.com/jackwener/opencli xueqiu implementation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: add Xueqiu to README platform tables, remove stale Instagram ref

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: fernando_jacob <f.jacob1996@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_channel_contracts.py

@@ -117,6 +117,7 @@ def test_channel_can_handle_contract():
         "linkedin": "https://www.linkedin.com/in/test",
         "weibo": "https://weibo.com/u/1749127163",
         "rss": "https://example.com/feed.xml",
+        "xueqiu": "https://xueqiu.com/S/SH600519",
         "exa_search": "https://example.com",
         "web": "https://example.com",
     }