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/fleet-option.nix

@@ -0,0 +1,9 @@
+# Flake-parts wrapper for fleet-option.nix
+{ inputs, self, lib, config, ... }:
+let
+  fleetModule = import ../fleet/fleet-option.nix { inherit inputs lib; };
+  inventoryModule = import ../inventory.nix { };
+in
+{
+  imports = [ fleetModule inventoryModule ];
+}