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

README.md

@@ -482,7 +482,8 @@ Uses **WebSocket** long connection — no public IP required.
       "appSecret": "xxx",
       "encryptKey": "",
       "verificationToken": "",
-      "allowFrom": ["ou_YOUR_OPEN_ID"]
+      "allowFrom": ["ou_YOUR_OPEN_ID"],
+      "groupPolicy": "open"
     }
   }
 }
@@ -491,6 +492,18 @@ Uses **WebSocket** long connection — no public IP required.
 > `encryptKey` and `verificationToken` are optional for Long Connection mode.
 > `allowFrom`: Add your open_id (find it in nanobot logs when you message the bot). Use `["*"]` to allow all users.
 
+**Group Chat Policy** (optional):
+
+| Option | Values | Default | Description |
+|--------|--------|---------|-------------|
+| `groupPolicy` | `"open"` | `"open"` | Respond to all group messages (backward compatible) |
+| | `"mention"` | | Only respond when @mentioned |
+
+> [!NOTE]
+> - `"open"`: Respond to all messages in all groups
+> - `"mention"`: Only respond when @mentioned in any group
+> - Private chats are unaffected (always respond)
+
 **3. Run**
 
 ```bash