fix: isStockSymbol misclassifies crypto symbols (BTC/ETH/SOL) as stock tickers

Critical bug: isStockSymbol('BTC') returned true because 'BTC' is 3
uppercase letters and the suffix check only catches 'BTCUSDT'-style pairs.
This caused crypto trade commands to be routed to Alpaca (stock trader)
instead of crypto exchanges.

Fix: add knownCryptoSymbols map with 60+ common crypto base symbols.
Check this map first before the heuristic letter-count check.

Also includes:
- Unit tests for isStockSymbol (crypto vs stock classification)
- Alpaca support in handleSyncBalance/handleClosePosition API handlers
- Cancel orphaned SL/TP orders before manual position close
- Clear peak PnL cache on position close (prevent stale data)
- Lighter FormatQuantity uses actual market precision instead of hardcoded 4

agent/tools_test.go

@@ -0,0 +1,65 @@
+package agent
+
+import "testing"
+
+func TestIsStockSymbol(t *testing.T) {
+	tests := []struct {
+		sym  string
+		want bool
+	}{
+		// Known crypto base symbols — must NOT be detected as stock
+		{"BTC", false},
+		{"ETH", false},
+		{"SOL", false},
+		{"BNB", false},
+		{"XRP", false},
+		{"DOGE", false},
+		{"ADA", false},
+		{"AVAX", false},
+		{"DOT", false},
+		{"LINK", false},
+		{"PEPE", false},
+		{"SHIB", false},
+		{"TRUMP", false},
+		{"USDT", false},
+		{"USDC", false},
+		{"W", false}, // single letter crypto
+
+		// Crypto pairs — must NOT be stock
+		{"BTCUSDT", false},
+		{"ETHUSDT", false},
+		{"SOLUSDT", false},
+		{"DOGEUSDT", false},
+
+		// Real stock tickers — must be detected as stock
+		{"AAPL", true},
+		{"TSLA", true},
+		{"NVDA", true},
+		{"MSFT", true},
+		{"GOOGL", true},
+		{"AMZN", true},
+		{"META", true},
+		{"AMD", true},
+		{"PLTR", true},
+		{"BA", true},
+		{"F", true},   // Ford — 1 letter
+		{"GM", true},  // 2 letters
+		{"JPM", true}, // 3 letters
+
+		// Mixed / edge cases
+		{"btc", false},  // lowercase crypto
+		{"aapl", true},  // lowercase stock (uppercased internally)
+		{"BTC123", false}, // not pure letters
+		{"123456", false}, // digits
+		{"", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.sym, func(t *testing.T) {
+			got := isStockSymbol(tt.sym)
+			if got != tt.want {
+				t.Errorf("isStockSymbol(%q) = %v, want %v", tt.sym, got, tt.want)
+			}
+		})
+	}
+}