Feature: Add position holding duration to AI decision context

Track and display how long each position has been held to help AI make better timing decisions.

**Implementation**:
- Added UpdateTime field to PositionInfo struct (decision/engine.go:26)
- Added positionFirstSeenTime map to AutoTrader for tracking (trader/auto_trader.go:60)
- Record opening time when position is created successfully:
  - executeOpenLongWithRecord: Records timestamp for long positions (trader/auto_trader.go:540-541)
  - executeOpenShortWithRecord: Records timestamp for short positions (trader/auto_trader.go:593-594)
- Fallback tracking in buildTradingContext for program restart scenarios (trader/auto_trader.go:386-392)
- Auto-cleanup closed positions from tracking map (trader/auto_trader.go:409-414)
- Display duration in user prompt with smart formatting:
  - Under 60 min: "持仓时长25分钟"
  - Over 60 min: "持仓时长2小时15分钟"

**Example Output**:
```
1. TAOUSDT LONG | 入场价435.5300 当前价433.1900 | 盈亏-0.54% | 杠杆20x | 保证金25 | 强平价418.1528 | 持仓时长2小时15分钟
```

**Benefits**:
- AI can see how long positions have been held
- Helps enforce minimum holding period (30-60 min) from system prompt
- Simple implementation with minimal overhead
- Auto-cleanup prevents memory leaks

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

decision/engine.go

@@ -23,6 +23,7 @@ type PositionInfo struct {
 	UnrealizedPnLPct float64 `json:"unrealized_pnl_pct"`
 	LiquidationPrice float64 `json:"liquidation_price"`
 	MarginUsed       float64 `json:"margin_used"`
+	UpdateTime       int64   `json:"update_time"` // 持仓更新时间戳（毫秒）
 }
 
 // AccountInfo 账户信息
@@ -335,10 +336,24 @@ func buildUserPrompt(ctx *Context) string {
 	if len(ctx.Positions) > 0 {
 		sb.WriteString("## 当前持仓\n")
 		for i, pos := range ctx.Positions {
-			sb.WriteString(fmt.Sprintf("%d. %s %s | 入场价%.4f 当前价%.4f | 盈亏%+.2f%% | 杠杆%dx | 保证金%.0f | 强平价%.4f\n\n",
+			// 计算持仓时长
+			holdingDuration := ""
+			if pos.UpdateTime > 0 {
+				durationMs := time.Now().UnixMilli() - pos.UpdateTime
+				durationMin := durationMs / (1000 * 60) // 转换为分钟
+				if durationMin < 60 {
+					holdingDuration = fmt.Sprintf(" | 持仓时长%d分钟", durationMin)
+				} else {
+					durationHour := durationMin / 60
+					durationMinRemainder := durationMin % 60
+					holdingDuration = fmt.Sprintf(" | 持仓时长%d小时%d分钟", durationHour, durationMinRemainder)
+				}
+			}
+
+			sb.WriteString(fmt.Sprintf("%d. %s %s | 入场价%.4f 当前价%.4f | 盈亏%+.2f%% | 杠杆%dx | 保证金%.0f | 强平价%.4f%s\n\n",
 				i+1, pos.Symbol, strings.ToUpper(pos.Side),
 				pos.EntryPrice, pos.MarkPrice, pos.UnrealizedPnLPct,
-				pos.Leverage, pos.MarginUsed, pos.LiquidationPrice))
+				pos.Leverage, pos.MarginUsed, pos.LiquidationPrice, holdingDuration))
 
 			// 使用FormatMarketData输出完整市场数据
 			if marketData, ok := ctx.MarketDataMap[pos.Symbol]; ok {