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/backthink_test.go

@@ -136,6 +136,63 @@ func TestBackthink_EventTrigger_NoPush(t *testing.T) {
 	}
 }
 
+func TestBackthink_SystemEventLowValueDowngraded(t *testing.T) {
+	t.Parallel()
+	// design/12 §6 策略一: a low-value system event on a public platform does not
+	// wake a full thinking pass — it is downgraded to a registration-only record.
+	thinker := &mockThinker{text: "should not be called"}
+	bs := bus.New()
+	b, _ := newBackthink(t, Config{}, Deps{
+		Bus:     bs,
+		Thinker: thinker,
+		Now:     func() time.Time { return dayAt() },
+	})
+
+	b.Start(context.Background())
+	defer func() { _ = b.Stop(context.Background()) }()
+
+	if err := bs.Publish(context.Background(), SubjectSystemEvent, SystemEvent{Summary: "member joined"}); err != nil {
+		t.Fatalf("Publish: %v", err)
+	}
+
+	eventually(t, 2*time.Second, func() bool {
+		recs, _ := b.List(context.Background(), 10)
+		return len(recs) >= 1 && recs[0].Trigger == SubjectSystemEvent
+	})
+	if got := thinker.count(); got != 0 {
+		t.Fatalf("thinker calls = %d, want 0 (low-value system event downgraded)", got)
+	}
+	recs, _ := b.List(context.Background(), 10)
+	if recs[0].ShouldPush {
+		t.Fatalf("ShouldPush = true for a downgraded system event")
+	}
+	if recs[0].PushResult.Route != RouteNone {
+		t.Fatalf("PushResult.Route = %q, want %q", recs[0].PushResult.Route, RouteNone)
+	}
+}
+
+func TestBackthink_SystemEventHighValueThinks(t *testing.T) {
+	t.Parallel()
+	// A high-importance (owner-relevant) system event still wakes a thinking pass —
+	// the denoise gate only downgrades low-value noise (design/12 §6 策略一).
+	thinker := &mockThinker{text: "important notice"}
+	bs := bus.New()
+	b, _ := newBackthink(t, Config{}, Deps{
+		Bus:     bs,
+		Thinker: thinker,
+		Now:     func() time.Time { return dayAt() },
+	})
+
+	b.Start(context.Background())
+	defer func() { _ = b.Stop(context.Background()) }()
+
+	if err := bs.Publish(context.Background(), SubjectSystemEvent, SystemEvent{Summary: "owner tagged", Import: ImportanceHigh}); err != nil {
+		t.Fatalf("Publish: %v", err)
+	}
+
+	eventually(t, 2*time.Second, func() bool { return thinker.count() >= 1 })
+}
+
 func TestBackthink_List_NewestFirst(t *testing.T) {
 	t.Parallel()
 	thinker := &mockThinker{text: "nothing to report"}