fix: cancel orphaned stop/TP orders on position close + guard zero SL/TP prices

Critical bugs fixed:
- executeCloseLong/CloseShort: cancel stop-loss and take-profit orders BEFORE closing
  position to prevent orphaned orders from triggering and creating unintended positions
- emergencyClosePosition: same fix for drawdown monitor's emergency close path
- Skip SetStopLoss/SetTakeProfit when AI returns price=0 (prevents exchange API errors)

Also includes:
- Add agent.Stop() to graceful shutdown sequence in main.go
- Remove unused variable suppression in Hyperliquid trader

trader/auto_trader_risk.go

@@ -112,6 +112,12 @@ func (at *AutoTrader) checkPositionDrawdown() {
 
 // emergencyClosePosition emergency close position function
 func (at *AutoTrader) emergencyClosePosition(symbol, side string) error {
+	// Cancel existing stop-loss and take-profit orders BEFORE closing position
+	// Critical: orphaned SL/TP orders could trigger after position is closed and create unintended positions
+	if err := at.trader.CancelStopOrders(symbol); err != nil {
+		logger.Warnf("  ⚠️ Emergency close: failed to cancel stop orders for %s: %v", symbol, err)
+	}
+
 	switch side {
 	case "long":
 		order, err := at.trader.CloseLong(symbol, 0) // 0 = close all