feat(channel,runtime): 批次2 后端 — 消息级角色归属进上下文 + IM 私聊随主会话角色 + 渠道级绑定

按 design/22 §2/§3/§4.4/§7（3/4/5）。
- ①「消息级角色归属」进入模型上下文（批次 1 只做存储+DTO）：历史消息的发送角色现在对模型可见，且**不把系统元数据塞进用户正文**（design/11 §10 红线）
- ② IM owner 私聊镜像解析出的角色 = **主会话绑定的角色**（§7-3）
- ③ 渠道级绑定：ChannelAccount 加 personaId（channels.json 读写/校验/脱敏沿用既有约定）+ 角色解析优先级 **会话 > 渠道 > 默认** + **群聊/陌生人强制默认角色**（§7-4）；admin 渠道配置端点支持 personaId
- Docs/api-client 版本 +1
- 验收：go build/vet/test 全绿 + linux/darwin 交叉编译

cmd/cyrene/persona_prompt.go

@@ -149,6 +149,24 @@ func (a *app) personaKnobs(personaID string) (loop.LoopKnobs, bool) {
 	}, true
 }
 
+// personaName resolves a role id to its display name for the model context's
+// speaker attribution (loop.Options.PersonaName, design/22 §7-5). The loop names a
+// speaking role as "「昔涟」（xilian）" instead of a bare id, so the model can follow
+// who said what after a session switched roles. An empty or unknown id resolves to
+// "" and the loop falls back to the id alone — attribution never disappears just
+// because a name could not be resolved, and nothing here is an identity or
+// permission input.
+func (a *app) personaName(personaID string) string {
+	if personaID == "" {
+		return ""
+	}
+	p, ok := a.personas.Persona(personaID)
+	if !ok || p == nil {
+		return ""
+	}
+	return strings.TrimSpace(p.Name)
+}
+
 // personaThinking maps the persona's `thinking` section onto the model-layer
 // knobs (design/01 §2.1 / design/04 §3.2). It returns (nil, "") when nothing
 // affects the request.