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/gateway/session_orchestrator.go

@@ -243,16 +243,27 @@ func (o *Orchestrator) submitInternal(ctx context.Context, sessionID string, mes
 	st.gen++
 	gen := st.gen
 	st.mode = ModeThinking
-	st.tail = o.boundedTail(messages)
+	// A fresh turn continues the session's conversation: the persisted transcript
+	// is seeded in front of the just-sent messages (design/15 §2.2/§4, design/22
+	// §7-5), and its assistant utterances keep the role that spoke them. The read
+	// happens here, before persistUserMessages below writes the new message, so the
+	// message just sent is never duplicated in the context. A store without history
+	// (or an empty session) leaves the turn on the caller's messages alone. It is a
+	// bounded indexed read (transcriptContextCount rows) under the state lock, which
+	// no running turn ever holds, so it cannot block a Submit behind a turn.
+	turn := o.startTurnMessages(ctx, sessionID, messages)
+	st.tail = o.boundedTail(turn)
 	runCtx, cancel := context.WithCancel(ctx)
 	st.cancel = cancel
 	o.mu.Unlock()
 
 	o.emitState(sessionID, EventSessionBusy, map[string]any{"sessionId": sessionID, "mode": ModeThinking})
+	// Only the messages that arrived now are persisted: the seeded transcript is
+	// already durable, and re-appending it would duplicate the whole conversation.
 	o.persistUserMessages(ctx, sessionID, messages, deviceID, msgMode, env.PersonaID)
 
 	if sync {
-		res, err := o.run(runCtx, sessionID, messages, env)
+		res, err := o.run(runCtx, sessionID, turn, env)
 		if err != nil {
 			o.log.Error("orchestrator turn", "err", err, "session", sessionID)
 		}
@@ -260,7 +271,7 @@ func (o *Orchestrator) submitInternal(ctx context.Context, sessionID string, mes
 		return SubmitResult{Accepted: true, Immediate: true, Err: err}, res
 	}
 
-	go o.goTurn(sessionID, gen, messages, env, runCtx, cancel, deviceID, msgMode)
+	go o.goTurn(sessionID, gen, turn, env, runCtx, cancel, deviceID, msgMode)
 	return SubmitResult{Accepted: true, Immediate: true}, nil
 }
 
@@ -297,7 +308,7 @@ func (o *Orchestrator) completeTurn(sessionID string, gen uint64, res *loop.Resu
 		return nil
 	}
 	if res != nil && res.Text != "" {
-		st.tail = o.boundedTail(concatMessages(st.tail, model.TextMessage(model.RoleAssistant, res.Text)))
+		st.tail = o.boundedTail(concatMessages(st.tail, assistantTailMessage(res.Text, personaID)))
 	}
 	if len(st.pending) > 0 {
 		next := st.pending[0]