Ignore the root build/ directory

The existing */build/* pattern only matches build/ inside a subdirectory,
so the xcodebuild -derivedDataPath output at the repo root kept showing
up as untracked.

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

.gitignore

@@ -1,6 +1,7 @@
 # Xcode
 .DS_Store
 */build/*
+/build/
 *.pbxuser
 !default.pbxuser
 *.mode1v3