Background

The check-environment-yml job in .github/workflows/ci.yml currently does its own pip install pyproject2conda and then invokes pyproject2conda yaml ... with a long argument list that is duplicated from the prek hook config in .pre-commit-config.yaml.

This means we have two pins for the same tool:

• .pre-commit-config.yaml → rev: v0.22.1 (GitHub tag, consumed by prek)
• .github/workflows/ci.yml → pyproject2conda>=0.22.1,<0.23 (PyPI version, consumed by pip install)

…and two copies of the argument list that have to stay in sync. PR #878 had to add an inline comment reminding future maintainers to bump both in lockstep, which is exactly the smell that motivates this issue.

Proposal

Replace the bespoke pip install + pyproject2conda yaml ... steps with a single call that runs the existing prek hook, e.g.:

- name: Run pyproject2conda via prek
  run: prek run pyproject2conda-yaml --all-files

Then diff the regenerated environment.yml against the committed one (the existing tail -n +10 / diff -u step can stay, or be replaced with git diff --exit-code -- environment.yml).

Benefits:

• Single source of truth for the pin (the prek hook rev).
• Single source of truth for the argument list (the prek hook args).
• Local prek run --all-files and CI exercise identical behaviour by construction.

Risks / things to check

• The prek hook writes to the workspace environment.yml; the current CI step writes to /tmp/environment.generated.yml. The diff strategy needs to flip from "compare two files" to "check git diff after running the hook," which is straightforward but is a behavioural change worth a careful review.
• The existing tail -n +10 skips a 9-line autogenerated header; we lose that workaround if we use git diff directly. Tracked separately as the header-fragility issue.
• Need to confirm prek is available on the runner without the j178/prek-action wrapper (or use that action here too).

Acceptance criteria

• check-environment-yml job calls a single prek-driven step.
• Bumping the pyproject2conda version requires editing exactly one file (.pre-commit-config.yaml).
• Job fails the build when environment.yml is out of sync, the same as today.

Context

Came out of an adversarial review of #878.