test: take the repo root from pytest so the ADR check can run from a fetched copy

[lesnik512]

What

`test_adr_citations.py` computed the repo root as `Path(file).parent.parent`. It now takes `pytestconfig.rootpath`.

Why

Pilot for replacing the 25 byte-identical copies of this test across the org with a fetch of this file from `main` (first consumer: eof-fixer). A fetched copy lives in a temp directory, so `file` points nowhere useful; `rootpath` is the repo pytest is run for.

For this repo nothing changes: `testpaths = ["tests"]` and the `pyproject.toml` ini section make rootpath the repo root. A consumer runs it as `pytest --rootdir=. `, which gives the same rootpath without pulling in the consumer's `addopts`.

Verified

• This repo: `uv run pytest`, 132 passed, 29 skipped.
• Against an eof-fixer checkout via `--rootdir=.`: 7 passed; a planted `docs/adr/0009-nope.md` citation fails with `eof_fixer/_planted.py cites docs/adr/0009-nope.md`.