roles: add role-based host composition in flake

- mkHost now takes a role list, roles resolve to ./roles/<name>.nix
- roles/core.nix = shared baseline (modules/)
- roles/k3s-server.nix = core + k3s (goyplex-vm)
- roles/hermes.nix = core + hermes (hermes-vm)
- host configuration.nix trimmed to hardware + boot + stateVersion
- optional hosts/<name>/custom.nix loads last for per-host overrides
- new hosts: one flake line + hosts/<name>/ dir

hosts/hermes-vm/configuration.nix

@@ -1,10 +1,11 @@
-{ pkgs, ... }:
+{
+  pkgs,
+  ...
+}:
 
 {
   imports = [
     ./hardware-configuration.nix
-    ../../modules/core
-    ../../modules/hermes
   ];
 
   boot.loader.systemd-boot.enable = true;