Add dynu_root_domain variable for domain import flow

infrastructure/terraform/dynu/imports.tf.example

@@ -1,11 +1,14 @@
-# Copy this file to imports.tf and adjust values after confirming the
-# published provider docs for resource type names and import ID formats.
+# Copy this file to imports.tf and adjust IDs after confirming the
+# published provider docs for import ID formats.
+# For dynu_domain, import ID is commonly the root domain name.
 
-# Example placeholder shape only:
-# import {
-#   to = dynu_dns_domain.lan_ddnsgeek_com
-#   id = "REPLACE_WITH_DYNU_DOMAIN_IMPORT_ID"
-# }
+import {
+  to = dynu_domain.lan_ddnsgeek_com
+  id = var.dynu_root_domain
+}
+
+# DNS record imports are intentionally examples only because the provider
+# requires explicit record_type/hostname in config before import.
 #
 # import {
 #   to = dynu_dns_record.grafana_lan_ddnsgeek_com