feat: add nufmt formatter

[gabyx]

• Adds nufmt formatter.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a new formatter module for nufmt and documents it in the README.

Changes:

• Introduce programs/nufmt.nix as a formatter module with optional --config support.
• Register nufmt in the README’s list of available formatters.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
programs/nufmt.nix	New Nix module wiring nufmt into the formatter framework and adding a configFile option.
README.md	Adds nufmt to the documented list of formatter modules.

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