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

prompts/adaptive.txt

@@ -330,26 +330,25 @@
 
 ## 仓位计算公式
 
-```
-仓位大小(USD) = 可用资金 × 风险预算 / 止损距离百分比
-仓位数量(Coins) = 仓位大小(USD) / 当前价格
-```
+**重要**：position_size_usd 是**名义价值**（包含杠杆），非保证金需求。
 
-**示例**：
-```
-账户净值：10,000 USDT
-风险预算：2%（信心度 90-95）
-止损距离：2%（50,000 → 49,000）
+**计算步骤**：
+1. **可用保证金** = Available Cash × 0.95 × Allocation %（预留5%给手续费）
+2. **名义价值** = 可用保证金 × Leverage
+3. **position_size_usd** = 名义价值（这是 JSON 中应填写的值）
+4. **Position Size (Coins)** = position_size_usd / Current Price
 
-仓位大小 = 10,000 × 2% / 2% = 10,000 USDT
-杠杆 5x → 保证金 2,000 USDT
-```
+**示例**：Available Cash = $500, Leverage = 5x, Allocation = 100%
+- 可用保证金 = $500 × 0.95 × 100% = $475
+- position_size_usd = $475 × 5 = **$2,375** ← JSON 中填写此值
+- 实际占用保证金 = $475，剩余 $25 用于手续费
 
-## 杠杆选择指南
+## 杠杆选择指引
 
-- 信心度 85-87: 3-5x 杠杆
-- 信心度 88-92: 5-10x 杠杆
-- 信心度 93-95: 10-15x 杠杆
+基于信心度的杠杆配置：
+- 信心度 <85 → 不开仓
+- 信心度 85-90 → 杠杆 1-3x，风险预算 1.5%
+- 信心度 90-95 → 杠杆 3-8x，风险预算 2%
 - 信心度 >95: 最高 20x 杠杆（谨慎）
 
 ## 风险控制原则