homelab-deploy: add NATS-based deployment system

Add homelab-deploy flake input and NixOS module for message-based
deployments across the fleet. Configure DEPLOY account in NATS with
tiered access control (listener, test-deployer, admin-deployer).
Enable listener on vaulttest01 as initial test host.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -21,6 +21,10 @@
       url = "git+https://git.t-juice.club/torjus/nixos-exporter";
       inputs.nixpkgs.follows = "nixpkgs-unstable";
     };
+    homelab-deploy = {
+      url = "git+https://git.t-juice.club/torjus/homelab-deploy?ref=master";
+      inputs.nixpkgs.follows = "nixpkgs-unstable";
+    };
   };
 
   outputs =
@@ -32,6 +36,7 @@
       alerttonotify,
       labmon,
       nixos-exporter,
+      homelab-deploy,
       ...
     }@inputs:
     let
@@ -58,6 +63,7 @@
         )
         sops-nix.nixosModules.sops
         nixos-exporter.nixosModules.default
+        homelab-deploy.nixosModules.default
         ./modules/homelab
       ];
       allSystems = [