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>

agent_reach/channels/__init__.py

@@ -22,6 +22,7 @@ from .wechat import WeChatChannel
 from .weibo import WeiboChannel
 from .xiaoyuzhou import XiaoyuzhouChannel
 from .v2ex import V2EXChannel
+from .xueqiu import XueqiuChannel
 
 
 
@@ -38,6 +39,7 @@ ALL_CHANNELS: List[Channel] = [
     WeiboChannel(),
     XiaoyuzhouChannel(),
     V2EXChannel(),
+    XueqiuChannel(),
     RSSChannel(),
     ExaSearchChannel(),
     WebChannel(),