Validate Github Repo Endpoint

Validate if a GitHub repository exists and is accessible.

This endpoint proxies the GitHub API check to avoid CORS issues from the frontend. Validates repository format and checks accessibility via GitHub API.

Args: repo: Repository in format "username/repo" github_token: Optional GitHub personal access token for private repos

Returns: { "valid": bool, "status": "valid" | "private" | "invalid" | "error", "message": str }