refactor: drop the reviewer rotation and rely on CODEOWNERS

[jaspermayone]

Removes assign-reviewer.yml and .github/dns-reviewers.txt. The DNS Managers team is asked on every pull request by CODEOWNERS, which is what actually gates the merge.

Why the rotation was not earning its keep

I built it to work around the free plan not having native team review assignment. Then two things came out:

The team gets asked either way. CODEOWNERS makes the team a required reviewer and GitHub will not let that request be dropped. I tried: DELETE /pulls/{n}/requested_reviewers with only team_reviewers returns 422 ("reviewers" wasn't supplied), and with the correct {"reviewers":[],"team_reviewers":[...]} payload it returns 200 and changes nothing. So the rotation could only ever add a second notification, never replace the first.

It was 90 lines of shell doing modular arithmetic over a text file that had to be kept in sync with team membership by hand. Two sources of truth for one question.

What changes in practice

Nothing about enforcement. require_code_owner_review plus CODEOWNERS is what blocks the merge, and that is untouched. pick the next reviewer was never a required check, so the ruleset needs no change.

What you lose is a named assignee. With two people that mostly means a pull request can sit while each of you assumes the other has it. CONTRIBUTING.md now says to speak up on the pull request when you pick one up or cannot get to it.

Adding a reviewer is now one step: add them to the team.

Note on merge order

This and #6 both touch docs/runbook.md, in different sections. Whichever lands second may want a rebase.

[github-actions]

octoDNS plan

This pull request changes no DNS records.

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Planned against main with a read-only Cloudflare token. Only the zone files at the repository root are included. Changes to config/, bin/ or .github/ are not in this plan, so review those by hand.