feat: dynamic stock search via Sina suggest API

- Add searchStock() using Sina's suggest API (type=11,31,41)
- Supports A-share, HK, and US markets dynamically
- No longer limited to hardcoded knownStocks map
- resolveStockCodeDynamic: tries static map first, then API fallback
- extractStockKeyword: smart keyword extraction from natural language
- New LLM tool 'search_stock': agent can proactively search stocks
- Top 3 search results auto-enriched with real-time quotes
- System prompts updated for both zh/en

agent/agent.go

@@ -294,6 +294,7 @@ func (a *Agent) buildSystemPrompt(lang string) string {
 
 ## 工具使用
 你可以调用以下工具来执行操作：
+- **search_stock** — 搜索股票（支持中文名、英文名、代码）。当用户提到你不认识的股票时，先用这个工具搜索。
 - **execute_trade** — 下单交易（做多/做空/平多/平空）。调用后会创建待确认订单，用户需回复"确认 trade_xxx"才会真正执行。
 - **get_positions** — 查看当前所有持仓
 - **get_balance** — 查看账户余额
@@ -338,6 +339,7 @@ func (a *Agent) buildSystemPrompt(lang string) string {
 
 ## Tools
 You can call these tools to take action:
+- **search_stock** — Search for stocks by name, ticker, or code. Covers A-share, HK, and US markets. Use when the user mentions an unknown stock.
 - **execute_trade** — Place a trade order (open_long/open_short/close_long/close_short). Creates a pending order that requires user confirmation.
 - **get_positions** — View all current open positions
 - **get_balance** — View account balance and equity
@@ -376,8 +378,8 @@ func (a *Agent) gatherContext(text string) string {
 		}
 	}
 
-	// A-share / stocks — try Sina Finance
-	stockCode, stockName := resolveStockCode(text)
+	// A-share / stocks — try Sina Finance (dynamic search as fallback)
+	stockCode, stockName := resolveStockCodeDynamic(text)
 	if stockCode != "" {
 		quote, err := fetchStockQuote(stockCode)
 		if err == nil && quote.Price > 0 {