Authenticate to the CircleCI API v2 with a Circle-Token and make a first request

Documented steps

1. Generate a Personal API Token in the CircleCI web app (User Settings > Personal API Tokens). Per the official API Developer's Guide: 'Currently, Personal API Tokens are the only supported tokens on API v2. Project Tokens are not currently supported on API v2.'
2. Use base URL https://circleci.com/api/v2 for all requests (CircleCI Server users substitute their own hostname).
3. Send the header 'Circle-Token: $CIRCLE_TOKEN' on every request.
4. Determine your project-slug. For GitHub OAuth or Bitbucket Cloud projects it is human-readable: vcs_type/org_name/repo_name, where vcs_type may be github/bitbucket or the short forms gh/bb (e.g. gh/my-org/my-repo).
5. For GitHub App, GitLab, and Bitbucket Data Center projects the slug is opaque: circleci/<org-id>/<project-id>. Docs state a project slug must be treated as an opaque string - do not hand-construct it; fetch it from the UI/API.
6. Make a first call: curl -X GET https://circleci.com/api/v2/project/{project-slug} --header 'Accept: application/json' --header 'Circle-Token: $CIRCLE_TOKEN' to confirm auth and read back the project's org-id/project-id.
7. A 401 means the token is invalid/missing; a 404 on the project call usually means the project-slug is wrong or CircleCI isn't set up for that repo yet.
8. Reference: https://circleci.com/docs/guides/toolkit/api-developers-guide/