ci: fetch the ADR citation check from .github instead of keeping a copy

[lesnik512]

What

Deletes tests/test_adr_citations.py (one of 25 byte-identical copies across the org) and adds a just adr-check recipe that fetches the file from modern-python/.github at main into a temp dir and runs it with pytest --rootdir=.. The lint job runs it once; it is not on the pytest matrix, since the check is interpreter-independent.

Same change as the pilot, modern-python/eof-fixer#55, which is merged and green on main.

Why

Tracking main rather than a pinned ref or a package is deliberate: one place to change the rule, no release step, no version bumps across 25 repos. The trade is that a change in .github takes effect everywhere at once, and a failing run cannot be reproduced against the file it saw unless the fetch is repeated while main is unchanged.

--rootdir=. makes the fetched file see this repo as the root without applying this repo's addopts (so the coverage gate does not fire on a one-file run). adr_check_source can be overridden to a file:// path to run a not-yet-merged version of the check.

The deleted copy is byte-identical to the file .github served before modern-python/.github#119, which changed only how the root is found. The rule this repo is checked against is unchanged.

Verified locally

just adr-check against .github main: 7 passed, rootdir this repo.

This repo

The lint job did not use just, so it gains extractions/setup-just@v4 to run the recipe; the rest of the job is unchanged. Verified with a plain venv holding only pytest, since psycopg2 does not build on the local machine; CI syncs the full groups as before.