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/channels.go

@@ -68,6 +68,25 @@ func (f *channelFactory) Build(platform string) ([]adapter.PlatformAdapter, erro
 	return buildPlatformAdapters(platform, accounts[platform], f.log), nil
 }
 
+// Persona returns the role bound to one channel account of a platform (design/22
+// §2 方案 b). It reads the same effective config Build does — channels.json wins,
+// env is the fallback — so the panel's save plus the bridge's hot reload stay the
+// single update path, and it is called once per adapter connect rather than per
+// message. An unknown platform/account, or an account without a personaId, returns
+// "" (no channel-level binding: the session binding or the global default applies).
+func (f *channelFactory) Persona(platform, accountID string) string {
+	accounts, warn := runtimeconfig.LoadChannels(f.path, f.fallback)
+	if warn != nil {
+		f.log.Warn("channels.json persona lookup fell back to env", "error", warn)
+	}
+	for _, acct := range accounts[platform] {
+		if strings.TrimSpace(acct.ID) == strings.TrimSpace(accountID) {
+			return strings.TrimSpace(acct.PersonaID)
+		}
+	}
+	return ""
+}
+
 // buildPlatformAdapters builds every account of one platform into an adapter
 // instance. A failed account build is skipped with a warn (never fatal), matching
 // the "no credentials → warn and continue" contract of the bridge.