refactor(natu): remove custom Nix linter

AGENTS.md

@@ -105,13 +105,10 @@
   Do not use `nix fmt` as a validation command until a formatter is added.
 - The `fix` app passes its current directory to Deadnix.
   Run `nix run .#fix` from the repository root so path exclusions match.
-- During the Nix refactor, address each issue that comes up by adding either a Natu rule or a new Nix style rule.
 - Git pathspec searches started in a subdirectory default to that subtree.
   Use `git -C <repository-root>` or top-anchored pathspecs for repository-wide history searches.
 - A `**/*.nix` Git pathspec does not include Nix files directly in its starting directory.
   Include both `*.nix` and `**/*.nix`, or use a recursive `*.nix` pathspec, when linting every Nix file.
-- Nix `let` bindings are recursive; their source order does not determine binding availability.
-  Treat Natu `let_order` as a readability rule, not an evaluation dependency.
 - Ariadne extraction must preserve the game-root season prefix for archived source.
   Different game installs can contain compiled scripts with the same internal path.
 - Clew owns Ren'Py extraction, preparation, and compiled-script AST export. Ariadne owns analysis over Clew-produced data.