chore: add build artifacts and PHPUnit cache to .gitignore

Ignore build/ directory (PHPStan cache and test reports) and
.phpunit.cache/ directory to prevent build artifacts from being
accidentally committed.

Author: houdaslassi

.gitignore

@@ -15,8 +15,12 @@ Thumbs.db
 
 # Testing
 .phpunit.result.cache
+.phpunit.cache/
 .pest.xml
 
+# Build artifacts
+/build/
+
 # Local docs (keep these out)
 # RELEASE.md - internal release guide
 # TODO.md - internal notes