Critical Fix:
- Restored stdio mode as default for supergateway/Claude Desktop compatibility
- Supergateway expects stdio communication, not HTTP server

New Features:
- Dual mode support: stdio and SSE
- main_stdio(): stdio mode (default, for uvx/supergateway)
- main_sse(): SSE HTTP server mode (for standalone use)
- Two entry points available:
  * bachai-data-analysis-mcp (stdio mode)
  * bachai-data-analysis-mcp-sse (SSE mode)

Usage:
- For supergateway/Claude Desktop: uvx bachai-data-analysis-mcp
- For standalone HTTP server: python main.py or bachai-data-analysis-mcp-sse

This version should work correctly with your supergateway setup.