fix: Refactor flake structure to properly use flake-parts

- Remove incorrect parts/fleet-data.nix import from flake.nix
- Create flake-parts wrappers for fleet-option.nix and users.nix
- Import inventory.nix through fleet-option wrapper
- Fix module argument passing (remove pkgs from mkFleet call)
- Move NixOS-specific modules out of flake-parts imports

This addresses the 'path does not exist' error but introduces infinite recursion that needs to be resolved.

parts/users.nix

@@ -0,0 +1,7 @@
+# Flake-parts wrapper for users.nix
+{ inputs, ... }:
+let
+  # Minimal pkgs just for shell paths - will be overridden in actual NixOS configs
+  pkgs = inputs.nixpkgs.legacyPackages.x86_64-linux;
+in
+import ../users.nix { inherit pkgs; }