feat: add http transport support

[gabeosx]

Summary

• add optional HTTP transport with health endpoint
• document HTTP usage and Docker flow
• silence stdio logging to avoid protocol corruption
• add Dockerfile and .dockerignore

Testing

• npm run build
• npm run lint
• docker build -t xero-mcp-server .
• MCP_TRANSPORT=http ./start-server.sh
• curl http://localhost:8080/healthz

[gabeosx]

I think this would be super helpful for those trying to use MCP clients like OpenAI Agent Builder which require HTTP transport, and do not support stdio.

[kitebuggy]

This would also solve the challenges around uploading attachments via stdio too. Nice one, has my vote!