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/variables.tf

@@ -52,3 +52,24 @@ variable "actions_token_1" {
   sensitive   = true
 }
 
+variable "homelab_deploy_listener_nkey" {
+  description = "NKey seed for homelab-deploy listeners"
+  type        = string
+  default     = "PLACEHOLDER"
+  sensitive   = true
+}
+
+variable "homelab_deploy_test_deployer_nkey" {
+  description = "NKey seed for test-tier deployer"
+  type        = string
+  default     = "PLACEHOLDER"
+  sensitive   = true
+}
+
+variable "homelab_deploy_admin_deployer_nkey" {
+  description = "NKey seed for admin deployer"
+  type        = string
+  default     = "PLACEHOLDER"
+  sensitive   = true
+}
+