feat(channels): add V2EX support via public API (zero-config, tier=0)

* feat(channels): add V2EX support via public API

V2EX provides a public JSON API that requires no authentication.
This PR adds:
- agent_reach/channels/v2ex.py: V2EXChannel (tier=0, zero-config)
  - can_handle() matches v2ex.com URLs
  - check() verifies API reachability via urllib (no extra deps)
- Register V2EXChannel in channels/__init__.py
- SKILL.md: add V2EX section with curl examples for hot topics,
  node browsing, topic detail, replies, and user info
- tests/test_channels.py: URL matching + mocked ok/warn check tests

V2EX API endpoints used:
  GET /api/v2/topics/hot          — hot topics
  GET /api/topics/show.json       — node topics / topic detail
  GET /api/replies/show.json      — topic replies
  GET /api/members/show.json      — user info

* feat(channels): expand V2EX channel with data-fetching methods

Add get_hot_topics, get_node_topics, get_topic, get_user, and search
methods to V2EXChannel using stdlib urllib only (no new dependencies).
Update unit tests and SKILL.md with Python call examples.

* feat(v2ex): add data fetching methods to V2EXChannel

agent_reach/channels/__init__.py

@@ -21,9 +21,10 @@ from .linkedin import LinkedInChannel
 from .wechat import WeChatChannel
 from .weibo import WeiboChannel
 from .xiaoyuzhou import XiaoyuzhouChannel
+from .v2ex import V2EXChannel
+
 
 
-# Channel registry
 ALL_CHANNELS: List[Channel] = [
     GitHubChannel(),
     TwitterChannel(),
@@ -36,6 +37,7 @@ ALL_CHANNELS: List[Channel] = [
     WeChatChannel(),
     WeiboChannel(),
     XiaoyuzhouChannel(),
+    V2EXChannel(),
     RSSChannel(),
     ExaSearchChannel(),
     WebChannel(),