Improve error messages and auto-generated slug naming

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

Author: Copilot

features/site-create.feature

@@ -169,7 +169,7 @@ Feature: Create a new site on a WP multisite
     When I try `wp site create --site-url=http://123.example.com`
     Then STDERR should be:
       """
-      Error: Could not derive a valid slug from the domain. Please provide --slug explicitly.
+      Error: Could not derive a valid slug from the domain (numeric-only or empty slugs are not allowed). Please provide --slug explicitly.
       """
     And the return code should be 1
 

src/Site_Command.php

@@ -471,7 +471,7 @@ public function create( $args, $assoc_args ) {
 
 					// Validate that the derived base is suitable for use as a slug
 					if ( empty( $base ) || is_numeric( $base ) ) {
-						WP_CLI::error( 'Could not derive a valid slug from the domain. Please provide --slug explicitly.' );
+						WP_CLI::error( 'Could not derive a valid slug from the domain (numeric-only or empty slugs are not allowed). Please provide --slug explicitly.' );
 					}
 				} else {
 					// For subdirectory installs, use the path as the base
@@ -483,7 +483,7 @@ public function create( $args, $assoc_args ) {
 					}
 					// If base is empty (root path), generate an auto slug
 					if ( empty( $base ) ) {
-						$base = 'auto-' . time();
+						$base = 'site-' . time();
 					}
 				}
 			} else {