Add issue reference strict mode

Some of our tooling seems to bail on commits with links as the `Fixes: ` trailer. Apply a strict mode to look for [github's syntax](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword) when these are present.

Return a non-0 status with a useful error (and ideally, parse a correct form to use to provide back to a PR submitter.)