Summary

• add a hooks.flake-follows hook backed by flake-edit follow
• expose a-kenji/flake-edit in the git-hooks tool set
• add hook config and behavior tests for the new hook

Motivation

flake-edit follow mutates flake.nix by adding missing follows declarations. That makes it a good fit for a pre-commit hook: it is a repository hygiene action.

Testing

Tested locally on x86_64-linux:

nix eval --show-trace --file ./default.nix run
nix build .#checks.x86_64-linux.hook-config-test
nix build .#checks.x86_64-linux.flake-follows-test
cd dev
nix flake check -L --show-trace --no-write-lock-file
nix flake check -L --show-trace \
  --override-input git-hooks/nixpkgs github:NixOS/nixpkgs/nixos-25.11