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/gateway_test.go

@@ -384,6 +384,33 @@ func TestWSUpgradeAndBroadcast(t *testing.T) {
 	}
 }
 
+func TestWsEventSessionIDField(t *testing.T) {
+	t.Parallel()
+	// When set, sessionId is marshalled so the client can filter by session.
+	set, err := json.Marshal(wsEvent{Type: "stream_end", SessionID: "web", Data: map[string]any{"stopReason": "stop"}})
+	if err != nil {
+		t.Fatalf("marshal set: %v", err)
+	}
+	if !strings.Contains(string(set), `"sessionId":"web"`) {
+		t.Fatalf("set frame = %s, want sessionId web", set)
+	}
+	// When empty, omitempty drops it so existing clients/tests see no change.
+	unset, err := json.Marshal(wsEvent{Type: "stream_end", Data: map[string]any{"stopReason": "stop"}})
+	if err != nil {
+		t.Fatalf("marshal unset: %v", err)
+	}
+	if strings.Contains(string(unset), "sessionId") {
+		t.Fatalf("unset frame = %s, want sessionId omitted", unset)
+	}
+	var ev wsEvent
+	if err := json.Unmarshal(unset, &ev); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if ev.SessionID != "" {
+		t.Fatalf("decoded SessionID = %q, want empty", ev.SessionID)
+	}
+}
+
 // readWS reads one text frame and decodes it into a wsEvent.
 func readWS(t *testing.T, conn *websocket.Conn) wsEvent {
 	t.Helper()