gitignore: ignore all of config/system/, not just two files

additional.php.save (editor backup) was untracked and unignored - one git add -A away from pushing config internals. settings.php holds credentials, so the whole directory is off limits.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -9,8 +9,10 @@
 .env
 .env.*
 !.env.example
-/config/system/settings.php
-/config/system/additional.php
+# The whole directory, not just the two known files: editor backups like
+# additional.php.save would otherwise slip into a careless `git add -A`
+# (settings.php holds credentials).
+/config/system/
 auth.json
 
 # --------------------------------------------------