Scope .gitignore to project tech stack

Move environment-specific entries (OS, editors, Claude) to the global
gitignore; keep only technology-specific and project-specific ignores here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: RISCfuture

.gitignore

@@ -1,13 +1,18 @@
-.DS_Store
+# Swift Package Manager
 /.build
 /Packages
-xcuserdata/
-DerivedData/
 .swiftpm/configuration/registries.json
 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+
+# Xcode user state
+xcuserdata/
+DerivedData/
+
+# netrc credentials
 .netrc
-.swift-format
 
+# SwiftLint remote config cache
+.swiftlint/RemoteConfigCache
 
-# SwiftLint Remote Config Cache
-.swiftlint/RemoteConfigCache
+# swift-format (downloaded in CI)
+.swift-format