fix: goroutine leak on shutdown, unsafe type assertions, redundant API calls

- Agent: add stopCh to cleanly stop chat-history-cleanup goroutine on Agent.Stop()
  (previously leaked a goroutine+ticker forever on every restart)
- Agent: add looksLikeStockQuery() guard to avoid hitting Sina search API on every
  single message — only calls external API when text contains stock-related content
  (saves ~200ms latency + API call on crypto-only queries)
- market/historical.go: safe type assertions in kline parsing (was bare .(float64)
  which panics on unexpected API responses), reuse HTTP client for connection pooling
- market/api_client.go: safe comma-ok type assertions for all kline field parsing
  (11 bare assertions → all guarded)
- trader/bybit: fix unsafe type assertion in CloseShort — pos["positionAmt"].(float64)
  could panic if field is nil/wrong type (critical: handles real money)

trader/bybit/trader_orders.go

@@ -154,7 +154,9 @@ func (t *BybitTrader) CloseShort(symbol string, quantity float64) (map[string]in
 		for _, pos := range positions {
 			side, _ := pos["side"].(string)
 			if pos["symbol"] == symbol && strings.ToLower(side) == "short" {
-				quantity = -pos["positionAmt"].(float64) // Short position is negative
+				if amt, ok := pos["positionAmt"].(float64); ok {
+					quantity = -amt // Short position is negative
+				}
 				break
 			}
 		}