External Links Check Fails on Rate Limit

Now that we're linking proto files in the docs, we're hitting GH rate limits without a token. The external links check isn't required to merge, but it now always fails. We've also already had to carve out a few links due to cloudflare protections. Worth discussing if this check is still worth automating.

Fixed by #7. Wired in GH token on CI, and added conditional check for user CLI based on presence of GITHUB_TOKEN env var.