FIX: Correct sed insert syntax in add_disable_wpcron_to_config

Problem: @ delimiter not valid for sed i/a commands, caused unknown command error
Solution: Use proper sed syntax with forward slash and literal newline after backslash

The i and a commands in sed require a literal newline after the backslash.
Fixed by using actual newlines in the here-doc style syntax.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

modules/website/wordpress/wordpress-cron-manager.sh

@@ -1589,12 +1589,14 @@ add_disable_wpcron_to_config() {
 
     # Try to insert before WordPress stop editing comment (proper convention)
     if grep -q "$WP_CONFIG_MARKER" "$wp_config" 2>/dev/null; then
-        # Use sed with @ delimiter to avoid escaping issues, insert before marker
-        sed -i.wpbak "@$WP_CONFIG_MARKER@i\\$new_define" "$wp_config"
+        # Use sed with proper newline syntax for insert command
+        sed -i.wpbak "/$WP_CONFIG_MARKER/i\\
+$new_define" "$wp_config"
         return 0
     elif grep -q "$WP_EDIT_START" "$wp_config"; then
         # Fallback: if no stop editing found, add after opening PHP tag
-        sed -i.wpbak "@$WP_EDIT_START@a\\$new_define" "$wp_config"
+        sed -i.wpbak "/$WP_EDIT_START/a\\
+$new_define" "$wp_config"
         return 0
     else
         # File format is unexpected