## 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