feat: IM式跨端实时同步——message_persisted 广播 + 前端按id去重追加

- history.Store 新增 AppendFull（返回含 id/createdAt 的完整消息），Append 签名不变
- session_orchestrator 加可空 OnPersisted；persistUserMessages/persistAssistantReply 落库后回调广播
- server.go newOrchestrator 接 OnPersisted → hub.broadcast message_persisted（thinking 剥除）
- 前端 ws.ts 加 message_persisted case（仅 activeSessionId 匹配才处理）；appendOrUpdateMessage 升级为 id 精确upsert+内容/时间归一
- 实测：双 WS，A stream 发消息，B 免刷新收到 user+assistant 各1条（count=2 PASS）
- 红线: 身份仍靠 actor 元数据，不靠提示词；用户/助手消息、本端/异端统一路径

internal/services/gateway/session_persist.go

@@ -14,9 +14,11 @@ import (
 // orchestrator runs unchanged without a store (tests / an unwired gateway).
 
 // persistUserMessages writes each RoleUser message of a submitted turn to the
-// history store so the raw user stream is durable before the turn resolves. It is
-// a no-op when no history store is wired. Rejected turns (empty, pending-full)
-// never reach it, so nothing vacuous is persisted.
+// history store so the raw user stream is durable before the turn resolves, then
+// notifies the transport so every browser of the same account can append it
+// without a manual refresh (design/16 §3.3, §4.2). It is a no-op when no history
+// store is wired. Rejected turns (empty, pending-full) never reach it, so nothing
+// vacuous is persisted.
 func (o *Orchestrator) persistUserMessages(ctx context.Context, sessionID string, messages []model.Message) {
 	if o.history == nil {
 		return
@@ -29,23 +31,30 @@ func (o *Orchestrator) persistUserMessages(ctx context.Context, sessionID string
 		if text == "" && image == "" {
 			continue
 		}
-		if err := o.history.Append(ctx, history.Message{SessionID: sessionID, Role: history.RoleUser, Content: text, Image: image}); err != nil {
+		full, err := o.history.AppendFull(ctx, history.Message{SessionID: sessionID, Role: history.RoleUser, Content: text, Image: image})
+		if err != nil {
 			o.log.Error("orchestrator persist user", "err", err, "session", sessionID)
+			continue
 		}
+		o.notifyPersisted(full)
 	}
 }
 
 // persistAssistantReply writes the final assistant text of a settled turn to the
-// history store. It is a no-op when no history store is wired or the turn
-// produced no text (interrupted/errored turns leave res nil). The write runs
-// under a fresh background context so a canceled turn context never aborts it.
+// history store, then notifies the transport (design/16 §3.3, §4.2). It is a
+// no-op when no history store is wired or the turn produced no text
+// (interrupted/errored turns leave res nil). The write runs under a fresh
+// background context so a canceled turn context never aborts it.
 func (o *Orchestrator) persistAssistantReply(sessionID string, res *loop.Result) {
 	if o.history == nil || res == nil || res.Text == "" {
 		return
 	}
-	if err := o.history.Append(context.Background(), history.Message{SessionID: sessionID, Role: history.RoleAssistant, Content: res.Text}); err != nil {
+	full, err := o.history.AppendFull(context.Background(), history.Message{SessionID: sessionID, Role: history.RoleAssistant, Content: res.Text})
+	if err != nil {
 		o.log.Error("orchestrator persist assistant", "err", err, "session", sessionID)
+		return
 	}
+	o.notifyPersisted(full)
 }
 
 // messageTextImage returns the concatenated text and the first image URL of a