The remote MCP endpoint https://mcp.exa.ai/mcp requires API key authentication
via x-api-key header. Without this, the connection times out waiting for auth.
This change:
- Reads EXA_API_KEY from environment variable
- Passes it as x-api-key header when available
- Updates RemoteMcpConfig type to support optional headers
Co-authored-by: Junho Yeo <i@junho.io>