Individual developers should execute "make reformat-code" or equivalent at each
commit; they can use a pre-commit hook if they want.

The commit also adds a CI workflow that verifies that the tip of a PR (or of
main) respects the clang-format rules.

…y disable it

This change has been done individually in order to manually inspect it, as
advised by https://clang.llvm.org/docs/ClangFormatStyleOptions.html#insertbraces:
    Setting this option to true could lead to incorrect code formatting due to
    clang-format’s lack of complete semantic information. As such, extra care
    should be taken to review code changes made by this option.