fix: Alpaca integration bugs — field mapping, market hours, stock symbol handling

- Fix critical bug: Alpaca GetBalance returns wrong field names (total_equity
  vs totalEquity) — auto_trader was getting 0 equity, breaking all decisions
- Fix critical bug: Alpaca GetPositions missing positionAmt/unRealizedProfit
  fields — positions invisible to trading AI
- Fix CancelAllOrders: was nuking ALL orders globally, now filters by symbol
- Implement GetClosedPnL: was returning nil, now returns filled sell orders
- Add IsMarketOpen: checks Alpaca clock endpoint for market hours
- Add market hours check in trading loop: skip cycles when US market closed
  (saves LLM tokens and prevents failed orders)
- Fix parseTradeCommand: 'BUY AAPL 10' no longer becomes 'AAPLUSDT'
- Fix toolGetMarketPrice: route stock symbols to Alpaca, crypto to others
- Add exchange field to toolGetPositions output for multi-exchange clarity

agent/trade.go

@@ -112,7 +112,8 @@ func parseTradeCommand(text string) *TradeAction {
 		return nil
 	}
 	symbol = words[1]
-	if !strings.HasSuffix(symbol, "USDT") {
+	// Only append USDT for crypto symbols, not stock tickers
+	if !isStockSymbol(symbol) && !strings.HasSuffix(symbol, "USDT") {
 		symbol += "USDT"
 	}
 
@@ -226,7 +227,10 @@ func formatTradeConfirmation(trade *TradeAction, lang string) string {
 		"close_short": "平空 (Close Short)",
 	}
 
-	symbol := strings.TrimSuffix(trade.Symbol, "USDT")
+	symbol := trade.Symbol
+	if strings.HasSuffix(symbol, "USDT") {
+		symbol = strings.TrimSuffix(symbol, "USDT")
+	}
 	actionName := actionNames[trade.Action]
 	if actionName == "" {
 		actionName = trade.Action
@@ -308,7 +312,10 @@ func (a *Agent) handleTradeConfirmation(ctx context.Context, userID int64, text,
 	}
 
 	trade.Status = "executed"
-	symbol := strings.TrimSuffix(trade.Symbol, "USDT")
+	symbol := trade.Symbol
+	if strings.HasSuffix(symbol, "USDT") {
+		symbol = strings.TrimSuffix(symbol, "USDT")
+	}
 	actionEmoji := "📈"
 	if strings.Contains(trade.Action, "short") {
 		actionEmoji = "📉"