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/runtime/history/store_test.go

@@ -76,6 +76,44 @@ func TestAppendAndListNewestFirst(t *testing.T) {
 	}
 }
 
+func TestAppendFullReturnsID(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	ctx := context.Background()
+
+	full, err := st.AppendFull(ctx, Message{SessionID: MainSessionID, Role: RoleUser, Content: "hi"})
+	if err != nil {
+		t.Fatalf("append full: %v", err)
+	}
+	if full.ID == "" {
+		t.Fatal("AppendFull did not assign an id")
+	}
+	if full.CreatedAt == 0 {
+		t.Fatal("AppendFull did not assign a created_at")
+	}
+	if full.SessionID != MainSessionID || full.Role != RoleUser || full.Content != "hi" {
+		t.Fatalf("AppendFull returned %+v, want the stored message unchanged plus id/created_at", full)
+	}
+
+	// The message is actually persisted under the returned id.
+	msgs, err := st.List(ctx, MainSessionID, 0, 50)
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(msgs) != 1 || msgs[0].ID != full.ID {
+		t.Fatalf("list = %+v, want exactly the persisted message id %q", msgs, full.ID)
+	}
+
+	// A caller-supplied id/timestamp is preserved, not regenerated.
+	preset, err := st.AppendFull(ctx, Message{ID: "preset-id", SessionID: MainSessionID, Role: RoleAssistant, Content: "ok", CreatedAt: 42})
+	if err != nil {
+		t.Fatalf("append full preset: %v", err)
+	}
+	if preset.ID != "preset-id" || preset.CreatedAt != 42 {
+		t.Fatalf("AppendFull did not preserve preset id/created_at: %+v", preset)
+	}
+}
+
 func TestListPaginationBefore(t *testing.T) {
 	t.Parallel()
 	st := newStore(t)