feat: add time stop rule for stale positions

Sell position if held for TIME_STOP_HOURS (default 24h) with less than
TIME_STOP_MIN_GAIN_PCT (default +3%) gain. Frees up capital for
fresh momentum opportunities.

Priority: trailing stop (-10%) checked first, then time stop.
Both thresholds configurable via .env.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

core/trader.py

@@ -6,6 +6,7 @@ import logging
 import os
 import threading
 import time
+from datetime import datetime
 from typing import Optional
 
 import pyupbit
@@ -21,7 +22,7 @@ PER_POSITION = MAX_BUDGET // MAX_POSITIONS  # 종목당 33만3천원
 
 _lock = threading.Lock()
 _positions: dict = {}
-# 구조: { ticker: { buy_price, peak_price, amount, invested_krw } }
+# 구조: { ticker: { buy_price, peak_price, amount, invested_krw, entry_time } }
 
 _upbit: Optional[pyupbit.Upbit] = None
 
@@ -73,6 +74,7 @@ def buy(ticker: str) -> bool:
                 "peak_price": current,
                 "amount": amount,
                 "invested_krw": order_krw,
+                "entry_time": datetime.now(),
             }
             logger.info(
                 f"[매수] {ticker} @ {current:,.0f}원 | "