feat(feishu): add global group mention policy

- Add group_policy config: 'open' (default) or 'mention'
- 'open': Respond to all group messages (backward compatible)
- 'mention': Only respond when @mentioned in any group
- Auto-detect bot mentions by pattern matching:
  * If open_id configured: match against mentions
  * Otherwise: detect bot by empty user_id + ou_ open_id pattern
- Support @_all mentions
- Private chats unaffected (always respond)
- Clean implementation with minimal logging

docs: update Feishu README with group policy documentation

nanobot/config/schema.py

@@ -47,6 +47,8 @@ class FeishuConfig(Base):
     react_emoji: str = (
         "THUMBSUP"  # Emoji type for message reactions (e.g. THUMBSUP, OK, DONE, SMILE)
     )
+    # Group chat settings
+    group_policy: Literal["open", "mention"] = "open"  # Group response policy (default: open for backward compatibility)
 
 
 class DingTalkConfig(Base):