Add SSE server mode with /healthz endpoint

[fedecia]

Summary

This PR adds an SSE (Server-Sent Events) server mode to the GitHub MCP Server, enabling HTTP-based transport in addition to the existing stdio mode.

Changes

New SSE Server Mode

• Added sse subcommand to start the server in SSE mode
• Uses the MCP SDK's built-in SSEHandler for the 2024-11-05 spec
• Configurable listen address via --sse-addr flag (default: :8080)

Health Endpoint

• Added /healthz endpoint for Kubernetes liveness/readiness probes
• Returns {"status":"ok"} with HTTP 200

Dockerfile Updates

• Changed default mode to SSE
• Exposes port 8080

Usage

# Start SSE server
github-mcp-server sse --sse-addr=:8080

# With Docker
docker run -p 8080:8080 -e GITHUB_PERSONAL_ACCESS_TOKEN=<token> ghcr.io/github/github-mcp-server

Testing

• Tested SSE endpoint returns proper text/event-stream content type
• Tested MCP initialize handshake over SSE
• Tested health endpoint returns 200 OK