feat: rewrite integrate skill to cover all 7 auth modes

[myeolinmalchi]

Summary

• Rewrite /integrate skill to support all 7 auth modes: standalone, credentials, oauth, email, oauth-direct, custom-jwt, api-key
• Accept backend server path as argument (/integrate /path/to/server) to directly configure auth handler
• Add interactive flow: select auth mode → provide mode-specific inputs → auto-configure both frontend and backend
• Split detailed auth handler code into separate reference files per mode category
• Add complete auth.py code, .env templates, langgraph.json config, and verification checklists for each mode