feat(engage): 群聊自主参与度引擎核心(design/18 P2a+P2b)

- persona 加 triggerWords(昔涟/Cyrene/有瑕的追忆/追忆)；Loader 解析
- light.go: shouldRespond 拆分 legacy 与 engage 路径；LightInput/LightDecision 扩展
- engage.go(新): EngageConfig + IDLE/PROBING/ACTIVE/COOLDOWN 状态机(按 platform:groupID 内存) +
  概率触发(可注入rand) + 角色名提词触发 + 活动期跟随(话题转移/连续无感/窗口) + 负面态度冷避
- engagejudge.go(新): NewLLMLightJudge，一次成本受控轻LLM判 值不值得回(不判身份)，
  解析失败/超时 fail-closed 静默
- channelbridge.go: Options(TriggerWords/PersonaPrompt/Engage/EngageFor/EngageRand) + 字段初始化
- wire.go: 接线 Light+TriggerWords+PersonaPrompt+Engage(默认概率0.10)
- 红线: 未触发/未命中概率绝不调LLM; 身份靠actor元数据; 敏感不落日志; 零新依赖
- 测试: engage_test.go(触发/状态机/冷避/成本受控/JUDGE输入) + persona triggerWords 用例
- 验收: go build/vet/test ./... 全绿

cmd/cyrene/wire.go

@@ -538,6 +538,14 @@ func (a *app) wireChannels() error {
 		// Optional review model prunes low-value queued messages near the per-adapter
 		// cap (design/12 §6 策略四). Nil adapter → the gate falls back to FIFO evict.
 		Review: reviewFor(a.modelAdapter, logger.WithSource(a.log, "services/channelbridge/review")),
+		// Light is the design/18 §3 participation judge: a cost-controlled light LLM
+		// decision on a triggered group/stranger message (role-name mention / probability
+		// / active window). Only an actual trigger feeds the group context.
+		Light:         engageLightFor(a.modelAdapter, logger.WithSource(a.log, "services/channelbridge/engage")),
+		TriggerWords:  a.defaultTriggerWords(),
+		PersonaPrompt: a.defaultInstructions(),
+		Engage:        channelbridge.DefaultEngageConfig(),
+		// EngageFor is left nil (P2a); per-group overrides arrive with P2c.
 		// Factory lets the panel save hot-replace a platform's adapters without a
 		// restart (design/04 §3.1).
 		Factory: factory,
@@ -742,3 +750,24 @@ func reviewFor(llm model.Adapter, log *slog.Logger) adapter.Reviewer {
 	}
 	return channelbridge.NewLLMReviewer(llm, log)
 }
+
+// engageLightFor builds the design/18 §3 participation judge, or nil when no model
+// adapter is available so the bridge's participation engine fails closed toward
+// silence (a triggered message without a judge is never answered).
+func engageLightFor(llm model.Adapter, log *slog.Logger) channelbridge.LightJudge {
+	if llm == nil {
+		return nil
+	}
+	return channelbridge.NewLLMLightJudge(llm, log)
+}
+
+// defaultTriggerWords returns the default persona's role-name reminder words
+// (design/18 §2) so the channel bridge knows which group messages cause a PROBING.
+func (a *app) defaultTriggerWords() []string {
+	if c, ok := a.personas.DefaultCharacter(); ok {
+		if p, ok := a.personas.Persona(c.PersonaID); ok {
+			return append([]string(nil), p.TriggerWords...)
+		}
+	}
+	return nil
+}