feat(nix): Add pre-commit hook to check flake input follows

[tlater-famedly]

No description provided.

[cursor]

PR Summary

Low Risk
Developer tooling only; no runtime app, auth, or data-path changes.

Overview
Adds a flake-follows local pre-commit hook that runs flake-edit follow on flake.nix and flake.lock so flake inputs stay recursively de-duplicated via follows.

The hook is declared in .pre-commit-config.yaml and wired into the Nix standards flake via new nix/nix/pre-commit-hooks.nix, which registers it on the root workspace and adds pkgs.flake-edit to prek-pre-commit.package.runtimePkgs. nix/nix/default.nix now imports that module alongside formatting.

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[RasmusRendal]

Works!