fix: Bitget 平仓必须传 holdSide 参数

- PlaceMarketOrder 新增 holdSide 参数
- 平仓时(closeLeg)根据 leg.Side 传 holdSide="long"/"short"
- 开仓传空字符串，不受影响
- 修复 BG 持续 'No position to close' (22002) 问题

exchange/bitget_trade.go

@@ -30,7 +30,7 @@ func NewBitgetTrade(apiKey, apiSecret, passphrase string) *BitgetTrade {
 	}
 }
 
-func (b *BitgetTrade) PlaceMarketOrder(side, symbol, size, tradeSide string) (string, error) {
+func (b *BitgetTrade) PlaceMarketOrder(side, symbol, size, tradeSide, holdSide string) (string, error) {
 	ts := fmt.Sprintf("%d", time.Now().UnixMilli())
 	method := "POST"
 
@@ -48,6 +48,10 @@ func (b *BitgetTrade) PlaceMarketOrder(side, symbol, size, tradeSide string) (st
 		"tradeSide":   tradeSide,
 		"size":        size,
 	}
+	// When closing, Bitget requires holdSide to identify which position to close
+	if tradeSide == "close" && holdSide != "" {
+		body["holdSide"] = holdSide
+	}
 	bodyJSON, _ := json.Marshal(body)
 
 	sign := b.sign(method, requestPath, ts, string(bodyJSON))