Ignore PHPUnit cache directory

Follow-up to Phase 7: .phpunit.cache is regenerated on every test run
and shouldn't be tracked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -8,3 +8,4 @@ composer.lock
 *.sqlite
 *.sqlite-journal
 /node_modules/
+/.phpunit.cache/