refactor: remove Douyin, Weibo and WeChat channels (#347)

All three had rotted past honest usability:
- Douyin's upstream (yzfly/douyin-mcp-server) is archived and required a
  4-step manual local-server setup nobody could complete
- Weibo depended on an unmaintained personal fork (mcp-server-weibo)
- WeChat full-article reading was increasingly blocked by anti-bot
  (#339) while doctor still advertised it as zero-config

Removes the channel files, installers, skill routing/trigger entries,
reference sections and README rows (zh+en). Honest counts: 13 platforms,
6 zero-config. They can return when maintained upstreams exist.

Follows the v1.4.0 precedent of removing Discord/Toutiao (#234).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_skill_command.py

@@ -114,7 +114,7 @@ class TestSkillCommand(unittest.TestCase):
             with open(target, encoding="utf-8") as f:
                 content = f.read()
             self.assertTrue(content.strip())
-            self.assertIn("WeChat Articles, Xiaoyuzhou Podcast", content)
+            self.assertIn("Xiaoyuzhou Podcast, LinkedIn", content)
             self.assertNotIn("搜推特", content)
             self.assertTrue(
                 os.path.exists(os.path.join(skill_parent, "agent-reach", "references"))