Add .gitignore and untrack user-specific files

Ignore macOS, Xcode user data, build output, SwiftPM and Reality
Composer Pro user files; remove already tracked xcuserdata and
rcuserdata from the index (files stay on disk).

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

.gitignore

@@ -0,0 +1,22 @@
+# macOS
+.DS_Store
+
+# Xcode user-specific data
+xcuserdata/
+*.xcuserstate
+*.xcscmblueprint
+*.xccheckout
+
+# Build output
+build/
+DerivedData/
+*.ipa
+*.dSYM.zip
+*.dSYM
+
+# Swift Package Manager
+.build/
+.swiftpm/
+
+# Reality Composer Pro user data
+*.rcuserdata