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/hyperliquid/trader_account.go

@@ -132,9 +132,6 @@ func (t *HyperliquidTrader) GetBalance() (map[string]interface{}, error) {
 			spotUSDCBalance, availableBalance)
 	}
 
-	// Suppress unused variable warning
-	_ = totalUnrealizedPnlAll
-
 	result["totalWalletBalance"] = totalWalletBalance       // Total assets (Perp + Spot + xyz) - unrealized
 	result["totalEquity"] = totalEquityCalculated           // Total equity = Perp AV + Spot + xyz AV
 	result["availableBalance"] = availableBalance           // Available balance (Perp + Spot if unified)