refactor: Rename breakEvenTriggerPercent to profitLockAfterTP1Percent for clarity

- Renamed config variable to accurately reflect behavior (locks profit, not breakeven)
- Updated log messages to say 'lock +X% profit' instead of misleading 'breakeven'
- Maintains backwards compatibility (accepts old BREAKEVEN_TRIGGER_PERCENT env var)
- Updated .env with new variable name and explanatory comment

Why: Config was named 'breakeven' but actually locks profit at entry ± X%
For SHORT at $141.51 with 0.3% lock: SL moves to $141.08 (not breakeven $141.51)
This protects remaining runner position after TP1 by allowing small profit giveback

Files changed:
- config/trading.ts: Interface + default + env parsing
- lib/trading/position-manager.ts: Usage + log message
- .env: Variable rename with migration comment

lib/trading/position-manager.ts

@@ -708,13 +708,13 @@ export class PositionManager {
       trade.currentSize = trade.positionSize * ((100 - this.config.takeProfit1SizePercent) / 100)
       const newStopLossPrice = this.calculatePrice(
         trade.entryPrice,
-        this.config.breakEvenTriggerPercent, // Use configured breakeven level
+        this.config.profitLockAfterTP1Percent, // Lock profit on remaining position
         trade.direction
       )
       trade.stopLossPrice = newStopLossPrice
       trade.slMovedToBreakeven = true
       
-      console.log(`🔒 SL moved to +${this.config.breakEvenTriggerPercent}% (${this.config.takeProfit1SizePercent}% closed, ${100 - this.config.takeProfit1SizePercent}% remaining): ${newStopLossPrice.toFixed(4)}`)
+      console.log(`🔒 SL moved to lock +${this.config.profitLockAfterTP1Percent}% profit (${this.config.takeProfit1SizePercent}% closed, ${100 - this.config.takeProfit1SizePercent}% remaining): ${newStopLossPrice.toFixed(4)}`)
       
       // CRITICAL: Cancel old on-chain SL orders and place new ones at updated price
       // BUT: Only if this is the ONLY active trade on this symbol