fix: Remove incorrect ./parts/fleet-data.nix import from flake.nix and use correct flake-parts structure

parts/formatter.nix

@@ -0,0 +1,9 @@
+# Formatter configuration for flake-parts
+{ ... }:
+{
+  perSystem =
+    { pkgs, ... }:
+    {
+      formatter = pkgs.nixfmt-rfc-style;
+    };
+}