It turns out there's no point in manually paginating; the Oktokit docs
recommend the use of auto_paginate instead.

This commit changes to using auto_paginate, and introduces an
integration test with HTTP record and replay courtesy VCR.

* Eliminate the use of `subject`.
* Use `value!` instead of `value_or`.

Requested in PR review; rationale explained at
https://thoughtbot.com/blog/lets-not.

Note that I'm setting this to the REDACTED value expected by config.

Also, this used to work locally - I must have accidentally killed the
`with_env` line while rebasing against main.

* Old hash syntax
* Redundant variable assignment

In addition to running specs, the default Rake task now also performs
style checking with standard.