chore: clean up untracked files and gitignore .claude/ directory

- Add make_superadmin utility scripts (list users, promote to super admin)
- Gitignore entire .claude/ directory (agents, settings, plans)
- Delete leftover tailwind-config-snippet.js and dual-mode plan doc

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -206,8 +206,8 @@ marimo/_static/
 marimo/_lsp/
 __marimo__/
 
-# Claude Code (local settings with permissions/tokens)
-.claude/settings.local.json
+# Claude Code (local config, agents, settings)
+.claude/
 
 # Railway CLI (local tooling)
 node_modules/