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/attic/headscale.nix

@@ -25,6 +25,10 @@ in
           client_secret_path = "/var/lib/headscale/client_secret";
           issuer = "https://nc.hoyer.xyz";
         };
+        policy = {
+          mode = "file";
+          path = toString ./headscale-policy.hujson;
+        };
       };
     };