feat(nixf-tidy): `--ci` flag

[Malix-Labs]

Is your feature request related to a problem? Please describe.

CI usually use exit code 1 to determine is a check is failing

Currently, nixf-tidy cannot be set to exit code 1 when lints are not passing

Describe the solution you'd like

A --ci flag in nixf-tidy to make it exit code 1 when lints are not passing

Additionally, it should be compatible with #841, and when set, should move its default severity to DS_Warn or DS_Error

Describe alternatives you've considered

nixf-diagnose

Additional context

• feat(nixf-tidy): flag --severity #841

[inclyc]

The core issue here is really about the role of nixf-tidy: originally it was designed for CI, but its output format wasn't user-friendly enough, so that responsibility was later taken over by nixf-diagnose.

So today, should nixf-diagnose be the one responsible for returning error codes in CI, while nixf-tidy remains just a lightweight C++ wrapper?

I don't know how real users see this—for instance, the only practical CI usage I'm aware of is nixpkgs using nixf-diagnose as part of treefmt; maybe that's already sufficient for CI? What's your actual use case—have you actually set up a CI pipeline based on nixf-tidy?

[Malix-Labs]

originally it [nixf-tidy] was designed for CI, but its output format wasn't user-friendly enough, so that responsibility was later taken over by nixf-diagnose

This was also what I understood from both projects indeed

So today, should nixf-diagnose be the one responsible for returning error codes in CI, while nixf-tidy remains just a lightweight C++ wrapper?

Well, that's a different concern;
Isn't that what libnixt is already?

Would it means nixf-tidy would stop being a CLI?
In which case, this issue would instantly be out of scope

What's your actual use case

Relying on nixf-tidy directly instead of its nixf-diagnose wrapper, since CI does not require user-friendly output format (it explicitly doesn't want it, even)