feat(channel): add WeCom customer service (wechatcom_kf) channel

Introduce a new channel that integrates with WeCom Customer Service
(微信客服), separate from the existing self-built WeCom app channel.

- Register channel type `wechatcom_kf` in factory, app loader and const
- Add config keys for token / secret / aes_key / port / cursor dir and
  the first-start history-skip switch; also expose corresponding env vars
- Implement channel, message and cursor store under channel/wechatcom_kf/

Co-authored-by: Cursor <cursoragent@cursor.com>

channel/channel_factory.py

@@ -27,6 +27,9 @@ def create_channel(channel_type) -> Channel:
     elif channel_type == "wechatcom_app":
         from channel.wechatcom.wechatcomapp_channel import WechatComAppChannel
         ch = WechatComAppChannel()
+    elif channel_type == const.WECHATCOM_KF:
+        from channel.wechatcom_kf.wechatcom_kf_channel import WechatComKfChannel
+        ch = WechatComKfChannel()
     elif channel_type == const.FEISHU:
         from channel.feishu.feishu_channel import FeiShuChanel
         ch = FeiShuChanel()