fix: lint cleanup and document group_trigger in example config

Add group_trigger entries to all channels in config.example.json that
support mention-only mode. Fix golines formatting in line.go, matrix.go,
config.go, and suppress nilerr for intentional silent skip in observeMessage.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/config.example.json

@@ -95,6 +95,9 @@
       "proxy": "",
       "allow_from": ["YOUR_USER_ID"],
       "use_markdown_v2": false,
+      "group_trigger": {
+        "mention_only": true
+      },
       "reasoning_channel_id": "",
       "streaming": {
         "enabled": true
@@ -115,6 +118,9 @@
       "app_id": "YOUR_QQ_APP_ID",
       "app_secret": "YOUR_QQ_APP_SECRET",
       "allow_from": [],
+      "group_trigger": {
+        "mention_only": true
+      },
       "reasoning_channel_id": ""
     },
     "maixcam": {
@@ -130,6 +136,9 @@
       "use_native": false,
       "session_store_path": "",
       "allow_from": [],
+      "group_trigger": {
+        "mention_only": false
+      },
       "reasoning_channel_id": ""
     },
     "feishu": {
@@ -139,6 +148,9 @@
       "encrypt_key": "",
       "verification_token": "",
       "allow_from": [],
+      "group_trigger": {
+        "mention_only": true
+      },
       "placeholder": {
         "enabled": true,
         "text": ["Thinking...", "Processing...", "Typing..."]
@@ -152,6 +164,9 @@
       "client_id": "YOUR_CLIENT_ID",
       "client_secret": "YOUR_CLIENT_SECRET",
       "allow_from": [],
+      "group_trigger": {
+        "mention_only": true
+      },
       "reasoning_channel_id": ""
     },
     "slack": {
@@ -159,6 +174,9 @@
       "bot_token": "xoxb-YOUR-BOT-TOKEN",
       "app_token": "xapp-YOUR-APP-TOKEN",
       "allow_from": [],
+      "group_trigger": {
+        "mention_only": true
+      },
       "reasoning_channel_id": ""
     },
     "matrix": {
@@ -186,6 +204,9 @@
       "channel_access_token": "YOUR_LINE_CHANNEL_ACCESS_TOKEN",
       "webhook_path": "/webhook/line",
       "allow_from": [],
+      "group_trigger": {
+        "mention_only": true
+      },
       "reasoning_channel_id": ""
     },
     "onebot": {
@@ -195,6 +216,9 @@
       "reconnect_interval": 5,
       "group_trigger_prefix": [],
       "allow_from": [],
+      "group_trigger": {
+        "mention_only": true
+      },
       "reasoning_channel_id": ""
     },
     "wecom": {

pkg/agent/loop.go

@@ -1448,7 +1448,7 @@ func (al *AgentLoop) observeMessage(ctx context.Context, msg bus.InboundMessage)
 			"channel": msg.Channel,
 			"error":   err.Error(),
 		})
-		return "", nil
+		return "", nil //nolint:nilerr // route resolution failure is not an error for observe-only messages
 	}
 
 	sessionKey := resolveScopeKey(route, msg.SessionKey)

pkg/channels/line/line.go

@@ -367,7 +367,9 @@ func (c *LINEChannel) processEvent(event lineEvent) {
 				"platform":    "line",
 				"source_type": event.Source.Type,
 			}
-			c.ObserveGroupMessage(c.ctx, observePeer, msg.ID, senderID, chatID, content, mediaPaths, observeMeta, observeSender)
+			c.ObserveGroupMessage(
+				c.ctx, observePeer, msg.ID, senderID, chatID, content, mediaPaths, observeMeta, observeSender,
+			)
 			return
 		}
 		content = cleaned

pkg/channels/matrix/matrix.go

@@ -728,7 +728,9 @@ func (c *MatrixChannel) handleMessageEvent(ctx context.Context, evt *event.Event
 				"room_id":  roomID,
 				"platform": "matrix",
 			}
-			c.ObserveGroupMessage(ctx, observePeer, evt.ID.String(), senderID, roomID, content, mediaPaths, observeMeta, sender)
+			c.ObserveGroupMessage(
+				ctx, observePeer, evt.ID.String(), senderID, roomID, content, mediaPaths, observeMeta, sender,
+			)
 			return
 		}
 		content = cleaned