Feature ask: enable github-mcp-server via config or command line argument

[lizy14]

Related to #1505
/mcp enable github-mcp-server works well for me. I hope I don't have to run it manually every time.

My use case: working on Azure DevOps repo, no GitHub remote, but my work benifits from GitHub MCP server:

• access to public GitHub repos that I depend on (e.g., Azure SDK)
• access to public information via Web Search (e.g., C# reference)

[lizy14]

$env:ADO_DETECTION = 'false' working for me as a (unsupported?) workaround