Using pre-commit hooks

[knoepfel]

We use a variety of formatting tools for the code in the phlex repository, which is written in C++, Python, YAML, CMake, Jsonnet, etc. Although there is a comment-based GitHub action that can apply formatting conventions to the pull request, we should consider incorporating https://pre-commit.com into our development workflow.

[olantwin]

We've made very positive experiences with pre-commit in SHiP, both for C++ and Python code.

I can prepare a PR with the first few hooks (e.g. clang-format, yaml syntax checking, ruff format, file endings, trailing whitespace) tomorrow.
I also saw that there's a hook for jsonnetfmt, but it seems unmaintained.

Would you also be interested in using the CI service? It's free for public github projects and very convenient.

[knoepfel]

Agreement at developer retreat: Using pre-commit would benefit our development workflow. Right now, the pre-commit steps should apply formatting changes only (and only for the changed files). Two points:

1. We do not think using the pre-commit CI is necessary right now.
2. The pre-commit configuration should stay in sync with other configurations used to achieve similar purposes; ideally, this would be a single point of maintenance within the repository.

[olantwin]

Agreement at developer retreat: Using pre-commit would benefit our development workflow. Right now, the pre-commit steps should apply formatting changes only (and only for the changed files).

When installed as a pre-commit hook, the hooks only run over the staged files.

Two points:

1. We do not think using the pre-commit CI is necessary right now.

OK.

2. The pre-commit configuration should stay in sync with other configurations used to achieve similar purposes; ideally, this would be a single point of maintenance within the repository.

At least the ruff and clang-format hooks reuse the same configuration as the CI, so no additional maintenance should be needed (apart from potentially updating the version every now and again).

I'll make a small PR to start with ruff format (no check) and clang-format. We can then expand from there.