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.

Why

Pilot of "shared test fetched from main" as the way to stop copying this file. The decision to track `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.

Depends on

modern-python/.github PR "test: take the repo root from pytest so the ADR check can run from a fetched copy". Until it merges, `main` still has the `file`-based root and the fetched copy scans the temp dir instead of this repo. The `adr-check` step here is only meaningful after that lands.

Verified locally

• `just adr_check_source=file://<.github worktree>/tests/test_adr_citations.py adr-check`: 7 passed.
• Same, with a planted broken citation in `eof_fixer/`: fails naming the file and the citation.
• `just lint-ci` clean; `just test-ci`: 37 passed, coverage 100%.