refactor: move tailscale join fully into cloud-init

Remove guest-agent enrollment workflow, pass TS auth key through Terraform variables/secrets, and run tailscale up with tag:k8s during cloud-init bootstrap alongside SSH key injection.

terraform/cloud-init.tf

@@ -2,9 +2,8 @@ data "template_file" "cloud_init_global" {
   template = file("${path.module}/files/cloud_init_global.tpl")
 
   vars = {
-    hostname       = "generic"
-    domain         = "home.arpa"
     SSH_KEY_PUBLIC = var.SSH_KEY_PUBLIC
+    TS_AUTHKEY     = var.TS_AUTHKEY
   }
 }