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

main.go

@@ -192,6 +192,10 @@ func main() {
 	}
 	logger.Info("✅ HTTP server stopped")
 
+	// Stop NOFXi agent (sentinel, brain, scheduler, cleanup goroutines)
+	nofxiAgent.Stop()
+	logger.Info("✅ NOFXi agent stopped")
+
 	// Stop all traders
 	traderManager.StopAll()
 	logger.Info("✅ System shut down safely")