feat: 公共平台最小化打扰(design/12 §6)——5条策略: 分级+轻思考+限流+审查剔噪

降噪A(策略一/二/五):
- channelbridge/light.go: 非owner分级门——小事件当系统消息登记(不进loop)、非owner轻响应(她判断回/不回,规则优先mention/关键词+轻LLM兜底fail-closed静默)
- backthink 触发降噪: SubjectSystemEvent 低价值不唤醒thinking(仅高价值/owner才think), iot/reminder不受影响
- quiet hours: 安静时段非owner无信号静默,被@才回(非硬性静默); config QUIET_START/END_HOUR 单源共用

限流B(策略三/四):
- adapter/flowgate.go: per-adapter 滑窗限流器+总上限+审查剔噪闸门(Admit两连:速率+上限/审查), owner私聊绕过
- channelbridge/flowgate.go: onMessage入站最前挂闸, 超限/被剔不占worker槽; 审查模型NewLLMReviewer复用model.Adapter(仅>80%阈值触发+跨分钟节流,失败回退FIFO,绝不判身份)
- config: ChannelAccount 加 GroupMsgPerMin(默认6)/MsgPerMin/MaxMessages(默认50), CHANNELS_JSON每账号, 向后兼容
- 被剔消息走OnRejected审计不静默丢

main wire接线: 加modelAdapter字段+Review注入(reviewFor nil-safe,无LLM回退FIFO)
全仓 go build/vet/test 绿(53包)+交叉编译, 零新依赖, 前端无涉

internal/services/backthink/trigger.go

@@ -17,6 +17,17 @@ const (
 	SubjectSystemEvent = "system.event"
 )
 
+// SystemEvent is the optional typed payload a caller may publish to
+// SubjectSystemEvent to convey a system notice. Import defaults to Normal. Under
+// design/12 §6 策略一, a low-value system notice (a join/leave ping, generic
+// platform noise) is downgraded to a registration-only record and does not wake a
+// full thinking pass; only a high-importance (owner-relevant) system event thinks,
+// so a busy public platform cannot trigger a flood of expensive reflections.
+type SystemEvent struct {
+	Summary string
+	Import  Importance
+}
+
 // Trigger describes what woke a thinking pass: the source subsystem, a
 // human-readable summary and the event's estimated importance.
 type Trigger struct {
@@ -48,7 +59,14 @@ func triggerForEvent(e bus.Event) Trigger {
 	case SubjectReminderTrigger:
 		return Trigger{Source: e.Subject, Summary: e.Subject, Import: ImportanceHigh}
 	case SubjectSystemEvent:
-		return Trigger{Source: e.Subject, Summary: e.Subject, Import: ImportanceNormal}
+		tr := Trigger{Source: e.Subject, Summary: e.Subject, Import: ImportanceNormal}
+		if sy, ok := e.Payload.(SystemEvent); ok {
+			if sy.Summary != "" {
+				tr.Summary = sy.Summary
+			}
+			tr.Import = sy.Import
+		}
+		return tr
 	default:
 		return Trigger{Source: e.Subject, Summary: e.Subject, Import: ImportanceNormal}
 	}