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/services/channelbridge/inbound.go

@@ -232,7 +232,13 @@ func (b *Bridge) runInboundTurn(ctx context.Context, msg adapter.UnifiedMessage,
 	// carried so the agent can see who is speaking (design/16 §4.6 §1-2). The
 	// resolved actor decides who is the owner vs a third party structurally — never
 	// from prompt text.
+	//
+	// The role this turn speaks as follows design/22 §2/§7-3/§7-4: the main session's
+	// binding when the owner's private chat is mirrored into it, otherwise the
+	// account's channel-level binding, otherwise the global default; a group or
+	// stranger conversation is forced onto the default role.
 	runEnv := loop.SessionEnv{ActorEnv: sess.Env(), MemWritable: sess.MemWritable}
+	runEnv.PersonaID = b.resolvePersona(ctx, sess, target, adapterName(a))
 	senderName := senderDisplayName(msg, actor)
 	loopMsgs := b.buildLoopMessages(ctx, text, runEnv, actor, senderName, msg)