Untrack .DS_Store and gitignore it

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CBuPQpo6XcLaBHfBwcJcRa

Author: tylerwillis

.DS_Store

@@ -7,3 +7,4 @@ project-log.md
 project-plan.md
 docs/explainer.html
 .claude/settings.json
+.DS_Store