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

internal/persona/loader/persona.go

@@ -86,6 +86,11 @@ type Persona struct {
 	Emotion      EmotionParams   `yaml:"emotion"`
 	Affection    AffectionParams `yaml:"affection"`
 	Visual       PersonaVisual   `yaml:"visual,omitempty"`
+	// TriggerWords are the role-name words that, when a public-platform group
+	// message mentions one, route the message into the participation PROBING
+	// stage even without a direct @ (design/18 §2 触发②). Each persona defines its
+	// own reminder words; they never imply identity (that stays structural).
+	TriggerWords []string `yaml:"triggerWords,omitempty"`
 }
 
 // EmotionConfig converts this persona's own fields into the params consumed by