Ignore .ruff_cache in .gitignore

Ruff writes its own .gitignore into the folder; this keeps it ignored if
that file is removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: PDowney

.gitignore

@@ -26,6 +26,7 @@ Thumbs.db
 .phpunit.cache
 .phpunit.result.cache
 .psalm/cache
+.ruff_cache/
 .stylelintcache
 __pycache__/