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>

terraform/vault/secrets.tf

@@ -92,6 +92,22 @@ locals {
       auto_generate = false
       data          = { token = var.actions_token_1 }
     }
+
+    # Homelab-deploy NKeys
+    "shared/homelab-deploy/listener-nkey" = {
+      auto_generate = false
+      data          = { nkey = var.homelab_deploy_listener_nkey }
+    }
+
+    "shared/homelab-deploy/test-deployer-nkey" = {
+      auto_generate = false
+      data          = { nkey = var.homelab_deploy_test_deployer_nkey }
+    }
+
+    "shared/homelab-deploy/admin-deployer-nkey" = {
+      auto_generate = false
+      data          = { nkey = var.homelab_deploy_admin_deployer_nkey }
+    }
   }
 }