[headtr1ck]

Description

Follow-up to #11753: agents still ignore AGENTS.md, e.g. asking for reruns of the flaky job in #11766.

• Add a root CLAUDE.md that imports AGENTS.md: Claude Code before v2.1.277 doesn't read AGENTS.md, and newer versions skip it if there is a CLAUDE.md in a parent directory or a CLAUDE.local.md.
• Move the test guidelines to xarray/tests/AGENTS.md so that other agents read them too, with xarray/tests/CLAUDE.md importing it, and reference them from AGENTS.md.
• Test guidelines: prefer parametrizing with the use_dask fixture or pytest.param(..., marks=requires_*) over separate tests, which avoids duplicate code and checks that numpy and dask give the same result. Mention that shared fixtures are in xarray/tests/conftest.py, drop the "Common patterns" section (it contradicted this and used a function that doesn't exist) and update the CI environment names.
• Replace the pytest.importorskip examples in the test guidelines: tests should be skipped at collection time with the requires_* decorators.
• Ask agents to always add type hints, and add them to the examples in the test guidelines.
• Use the pixi environments of the CI instead of uv for running tests and mypy, and pixi exec pre-commit (the pre-commit pixi task always succeeds).

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude Code with Opus 5.5

🤖 Generated with Claude Code

[headtr1ck]

@dcherian I noticed that you seem to be against parametrizing tests e.g. for with and without dask (as per your previous instructions in the agents file).
Is there a reason for it?
I find that this would just duplicate code for basically no benefit.

Is the approach with using the use_dask fixture ok for you?

[dcherian]

Uhh... i withdraw that objection. I prefer not doing the has_dask stuff, can be hard to see at a first glance (I think that's what prompted that edit to CLAUDE.md). I think the parametrize is OK.