feat: SSH连接池串行化 + Polymarket/数据源统一走连接池

- 新增ssh_proxy_pool.py: 全局单例连接池，队列串行化SSH请求
- stockstats_utils.py: _fetch_via_ssh改用连接池
- polymarket.py: 跳过本地直接走SSH连接池
- 消除并发SSH资源竞争问题

tradingagents/dataflows/stockstats_utils.py

@@ -43,19 +43,9 @@ def _get_ssh_config():
 
 
 def _fetch_via_ssh(url: str, timeout: int = 30) -> str:
-    """Fetch a URL through the foreign server via SSH tunnel."""
-    host, port, password = _get_ssh_config()
-    cmd = [
-        "sshpass", "-p", password,
-        "ssh", "-o", "StrictHostKeyChecking=no",
-        "-o", "ConnectTimeout=15",
-        "-p", str(port), f"root@{host}",
-        f'curl -s "{url}" -H "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36"'
-    ]
-    result = subprocess.run(cmd, capture_output=True, text=True, timeout=timeout)
-    if result.returncode != 0:
-        raise RuntimeError(f"SSH fetch failed: {result.stderr}")
-    return result.stdout
+    """Fetch a URL through the foreign server via SSH tunnel (uses connection pool)."""
+    from .ssh_proxy_pool import ssh_proxy_fetch
+    return ssh_proxy_fetch(url, timeout)
 
 
 def _yf_download_via_proxy(symbol: str, start: str, end: str) -> pd.DataFrame: