feat: add Weibo channel via mcp-server-weibo (#107)

Adds WeiboChannel integrating qinyuanpei/mcp-server-weibo (34+ stars,
MIT, 10 tools) via mcporter. Addresses #75.

Files changed (3 files, +57 lines):
- agent_reach/channels/weibo.py — new channel (54 lines)
- agent_reach/channels/__init__.py — register WeiboChannel
- tests/test_channel_contracts.py — add weibo URL sample

Co-authored-by: AICodeLion <AICodeLion@users.noreply.github.com>

agent_reach/channels/__init__.py

@@ -20,6 +20,7 @@ from .douyin import DouyinChannel
 from .linkedin import LinkedInChannel
 from .bosszhipin import BossZhipinChannel
 from .wechat import WeChatChannel
+from .weibo import WeiboChannel
 
 
 # Channel registry
@@ -34,6 +35,7 @@ ALL_CHANNELS: List[Channel] = [
     LinkedInChannel(),
     BossZhipinChannel(),
     WeChatChannel(),
+    WeiboChannel(),
     RSSChannel(),
     ExaSearchChannel(),
     WebChannel(),