feat(headscale): add ACL policy, isolate mx, make mx an exit node

Introduces a headscale ACL policy (file-mode) plus matching client config:

- New systems/x86_64-linux/attic/headscale-policy.hujson:
  * tag:llm restricts a node to talking only to halo:8000
  * all other harald@ nodes have full mesh access to each other
  * harald@ nodes can route internet traffic via approved exit nodes
  * autoApprovers.exitNode = [tag:llm] auto-approves the exit route
    advertised by any tag:llm node (currently mx)

- attic headscale.nix: wire policy.mode = "file" / policy.path to
  the .hujson above.

- mx default.nix: enable useRoutingFeatures = "server" (needed for IP
  forwarding) and add extraSetFlags = ["--advertise-exit-node"] so the
  flag is reapplied on every activation, not just initial login.

Operational steps after deploy:
  headscale nodes tag -i 10 -t tag:llm

systems/x86_64-linux/mx/default.nix

@@ -26,7 +26,11 @@
     ./users.nix
   ];
 
-  services.tailscale.enable = true;
+  services.tailscale = {
+    enable = true;
+    useRoutingFeatures = "server";
+    extraSetFlags = [ "--advertise-exit-node" ];
+  };
 
   metacfg = {
     services.nginxBase.enable = true;