feat(halo): serve multiple llama models via models.ini preset

Replace the per-model llama-server units with a single service that
uses llama-server's --models-preset (models.ini) and --models-max 2,
so the 35B-A3B and 27B models are loaded on demand from one config.

Drop the now-redundant 27B / 27B-MTP / coder-next variant files and
the unused CacheDirectory + slot-save-path KV-slot handling.

systems/x86_64-linux/halo/default.nix

@@ -10,8 +10,7 @@ with lib.metacfg;
     ./hardware-configuration.nix
     #./xremap.nix
     ./wyoming.nix
-    #./llama-server-coder-next.nix
-    ./llama-server-27B-MTP.nix
+    ./llama-server.nix
   ];
 
   boot.lanzaboote.pkiBundle = "/var/lib/sbctl";