feat(history): 会话消息持久化 + 主/次级会话（design/16）

- runtime/history: Store + HistoryStore 接口（Append/List 游标分页/Recent/DeleteSession/NewSubSession/ListSessions），主会话固定 web、次级 web:<uuid>
- db 迁移 0003: chat_sessions + chat_messages + idx_chat_msgs_session_created（事务幂等）
- gateway: GET/POST /sessions、GET /sessions/{id}/messages 热分页、DELETE /sessions/{id}（主不可删 403）；Deps.History 可空注入
- 编排层落库: session_persist.go（submitInternal 写用户消息、completeTurn 写助手消息，拒绝的消息不落库）
- WS: wsEvent 加 sessionId 定向 + history 回放帧（握手上行后回放主会话最新 50 条，thinking 剥离）
- wire.go 装配 History 到 gateway.Deps.History
- 端到端实测: POST /sessions 建次级 201 / GET /sessions 列表 / POST /chat 后 /messages 读到 user+assistant / DELETE 次级成功 / DELETE 主会话 403

internal/services/gateway/rest.go

@@ -175,8 +175,15 @@ func (s *Server) handleChat(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, chatResponse{Reply: res.Text, Steps: res.Steps, StopReason: res.StopReason})
 }
 
-// handleListSessions returns the known sessions (or [] when no store is wired).
+// handleListSessions returns the persistent chat-history sessions (main + sub) when a
+// history store is wired, otherwise falls back to the channel-session view. The fallback
+// keeps the pre-design/16 contract: a server built without history still lists the known
+// channel sessions (or [] when no store is wired).
 func (s *Server) handleListSessions(w http.ResponseWriter, r *http.Request) {
+	if s.history != nil {
+		s.handleHistorySessions(w, r)
+		return
+	}
 	out := make([]SessionSummary, 0)
 	if s.sessions != nil {
 		list, err := s.sessions.List(r.Context())