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.go

@@ -87,6 +87,9 @@ type SessionMeta struct {
 type HistoryStore interface {
 	// Append persists msg, assigning an id and timestamp when the caller omitted them.
 	Append(ctx context.Context, msg Message) error
+	// AppendFull persists msg like Append but returns the complete stored message
+	// (with the assigned id/timestamp) so a caller can broadcast it for cross-end sync.
+	AppendFull(ctx context.Context, msg Message) (Message, error)
 	// List returns up to limit messages for sessionID, newest first. before is a
 	// millisecond timestamp cursor: when nonzero, only messages older than before are
 	// returned (inclusive-free, matching the "before" bound on the frontend).
@@ -144,12 +147,28 @@ func NewStore(ctx context.Context, db *sql.DB, log *slog.Logger) (*Store, error)
 }
 
 // Append persists msg, assigning a fresh id and a now-ms CreatedAt when the caller left
-// either empty/zero. It returns the error wrapped with the operation name.
+// either empty/zero. It returns the error wrapped with the operation name. Callers that
+// need the generated id/timestamp back should use AppendFull instead.
 func (s *Store) Append(ctx context.Context, msg Message) error {
+	_, err := s.insert(ctx, msg)
+	return err
+}
+
+// AppendFull persists msg like Append but returns the complete stored message — the id
+// and CreatedAt the store assigned when the caller left them empty/zero — so a caller can
+// broadcast it for cross-end realtime sync (design/16 §4.2) without a second read.
+func (s *Store) AppendFull(ctx context.Context, msg Message) (Message, error) {
+	return s.insert(ctx, msg)
+}
+
+// insert is the shared Append/AppendFull body. It assigns a fresh id and a now-ms
+// CreatedAt when the caller left either empty/zero, then inserts the row and returns the
+// resulting message.
+func (s *Store) insert(ctx context.Context, msg Message) (Message, error) {
 	if msg.ID == "" {
 		id, err := newID()
 		if err != nil {
-			return fmt.Errorf("history: append id: %w", err)
+			return Message{}, fmt.Errorf("history: append id: %w", err)
 		}
 		msg.ID = id
 	}
@@ -162,9 +181,9 @@ func (s *Store) Append(ctx context.Context, msg Message) error {
 		msg.ID, msg.SessionID, msg.Role, msg.Content, msg.Mode, msg.Mood,
 		msg.Image, msg.Thinking, msg.CreatedAt)
 	if err != nil {
-		return fmt.Errorf("history: append %s to %s: %w", msg.Role, msg.SessionID, err)
+		return Message{}, fmt.Errorf("history: append %s to %s: %w", msg.Role, msg.SessionID, err)
 	}
-	return nil
+	return msg, nil
 }
 
 // List returns up to limit messages for sessionID, newest first. When before is nonzero