fix(margin): correct position sizing formula to prevent insufficient margin errors

## Problem
AI was calculating position_size_usd incorrectly, treating it as margin requirement instead of notional value, causing code=-2019 errors (insufficient margin).
## Solution
### 1. Updated AI prompts with correct formula
- **prompts/adaptive.txt**: Added clear position sizing calculation steps
- **prompts/nof1.txt**: Added English version with example
- **prompts/default.txt**: Added Chinese version with example
**Correct formula:**
1. Available Margin = Available Cash × 0.95 × Allocation % (reserve 5% for fees)
2. Notional Value = Available Margin × Leverage
3. position_size_usd = Notional Value (this is the value for JSON)
**Example:** $500 cash, 5x leverage → position_size_usd = $2,375 (not $500)
### 2. Added code-level validation
- **trader/auto_trader.go**: Added margin checks in executeOpenLong/ShortWithRecord
- Validates required margin + fees ≤ available balance before opening position
- Returns clear error message if insufficient
## Impact
- Prevents code=-2019 errors
- AI now understands the difference between notional value and margin requirement
- Double validation: AI prompt + code check
## Testing
- ✅ Compiles successfully
- ⚠️ Requires live trading environment testing

trader/auto_trader.go

@@ -626,6 +626,27 @@ func (at *AutoTrader) executeOpenLongWithRecord(decision *decision.Decision, act
 	actionRecord.Quantity = quantity
 	actionRecord.Price = marketData.CurrentPrice
 
+	// ⚠️ 保证金验证：防止保证金不足错误（code=-2019）
+	requiredMargin := decision.PositionSizeUSD / float64(decision.Leverage)
+
+	balance, err := at.trader.GetBalance()
+	if err != nil {
+		return fmt.Errorf("获取账户余额失败: %w", err)
+	}
+	availableBalance := 0.0
+	if avail, ok := balance["availableBalance"].(float64); ok {
+		availableBalance = avail
+	}
+
+	// 手续费估算（Taker费率 0.04%）
+	estimatedFee := decision.PositionSizeUSD * 0.0004
+	totalRequired := requiredMargin + estimatedFee
+
+	if totalRequired > availableBalance {
+		return fmt.Errorf("❌ 保证金不足: 需要 %.2f USDT（保证金 %.2f + 手续费 %.2f），可用 %.2f USDT",
+			totalRequired, requiredMargin, estimatedFee, availableBalance)
+	}
+
 	// 设置仓位模式
 	if err := at.trader.SetMarginMode(decision.Symbol, at.config.IsCrossMargin); err != nil {
 		log.Printf("  ⚠️ 设置仓位模式失败: %v", err)
@@ -685,6 +706,27 @@ func (at *AutoTrader) executeOpenShortWithRecord(decision *decision.Decision, ac
 	actionRecord.Quantity = quantity
 	actionRecord.Price = marketData.CurrentPrice
 
+	// ⚠️ 保证金验证：防止保证金不足错误（code=-2019）
+	requiredMargin := decision.PositionSizeUSD / float64(decision.Leverage)
+
+	balance, err := at.trader.GetBalance()
+	if err != nil {
+		return fmt.Errorf("获取账户余额失败: %w", err)
+	}
+	availableBalance := 0.0
+	if avail, ok := balance["availableBalance"].(float64); ok {
+		availableBalance = avail
+	}
+
+	// 手续费估算（Taker费率 0.04%）
+	estimatedFee := decision.PositionSizeUSD * 0.0004
+	totalRequired := requiredMargin + estimatedFee
+
+	if totalRequired > availableBalance {
+		return fmt.Errorf("❌ 保证金不足: 需要 %.2f USDT（保证金 %.2f + 手续费 %.2f），可用 %.2f USDT",
+			totalRequired, requiredMargin, estimatedFee, availableBalance)
+	}
+
 	// 设置仓位模式
 	if err := at.trader.SetMarginMode(decision.Symbol, at.config.IsCrossMargin); err != nil {
 		log.Printf("  ⚠️ 设置仓位模式失败: %v", err)