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

internal/config/config.go

@@ -297,6 +297,14 @@ type ChannelAccount struct {
 	// value disables the cap; a positive value pins it. When the pending set nears
 	// this cap the review model prunes low-value messages.
 	MaxMessages int `json:"maxMessages,omitempty"`
+	// PersonaID is the role this account answers as (design/22 §2 方案 b: 渠道级绑定).
+	// It is a persona id from the persona library, not a display name. It only
+	// provides the role for the owner's private chat with this account and only when
+	// the main session binds no role of its own: a session binding wins over it, the
+	// global default (owner.yaml defaultPersona) is the final fallback, and a
+	// group/stranger conversation is always forced onto the default role
+	// (design/22 §7-4). Empty means "no channel-level binding".
+	PersonaID string `json:"personaId,omitempty"`
 }
 
 // Load reads configuration, merging an optional .env file (development only)