Feature Request: `tidy` Command

[Malix-Labs]

This is a meta issue that regroups my unfulfilled needs about nix toolings so far:

• duplicate - duplicate assignments check: check for occurrences of duplicate similar value assignments (e.g. hardware.nvidia.open = true while the default already sets it), this would permit configs to rely more on upstream and sane defaults instead of duplicated hardcodings
• stateVersion - stateVersion check for outdated options and updater: a command that would check for nix, nixos, and home manager's currently set stateVersion and check in upstream (including flakes inputs) for outdated options (example: default = lib.versionAtLeast config.system.stateVersion "26.05"), with a flag to update the currently set stateVersion to the latest safe value
  https://github.com/search?type=code&q=org:NixOS+stateVersion
• nixConfig - nixConfig inheritance prompting: a command that would loop for every flake inputs for a nixConfig merge into nix.conf (similar to how the current nix 3 unified cli proposes to merge the current flake's nixConfig into nix.conf), with a --recursive behavior flag (loop for flake inputs nixConfig of flake inputs... recursively)
  also see nix: add recursive nixConfig support for flake inputs NixOS/nix#15584 which I authored and proposes a similar idea to the upstream nix 3 unified cli.
  (note: I am aware that @NotAShelf explicitly dislike flake's nixConfig, also see Handle nixConfig in flakes #70).