docs(wechatcom_kf): tidy README and hide cursor dir from config

- Clarify Secret retrieval (must tap "查看" on admin's phone, not copy)
- Update WeCom customer-service binding section to point to the
  "接入链接" UI (copy link / generate QR code)
- Drop developer-only asides (wechatcomapp_secret / port collision
  notes, internal sections about cursor persistence, channel runtime
  differences, multi-kf-account support)
- Stop exposing `wechatcom_kf_cursor_dir` as a user config; cursor file
  is now fixed under `tmp/`, which is an internal implementation detail.

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

config.py

@@ -157,7 +157,6 @@ available_setting = {
     "wechatcom_kf_port": 9888,  # 微信客服回调服务端口
     "wechatcom_kf_secret": "",  # 微信客服应用的secret
     "wechatcom_kf_aes_key": "",  # 微信客服回调aes_key
-    "wechatcom_kf_cursor_dir": "tmp",  # 拉取消息的cursor持久化目录(相对项目根)
     # 飞书配置
     "feishu_port": 80,  # 飞书bot监听端口，仅webhook模式需要
     "feishu_app_id": "",  # 飞书机器人应用APP Id