Agent Skills: authentication

Load PROACTIVELY when task involves user identity, login, or access control. Use when user says \"add authentication\", \"set up login\", \"add OAuth\", \"protect these routes\", \"implement RBAC\", or \"add sign-up\". Covers session management, JWT tokens, OAuth2 flows, password reset, email verification, protected route middleware, role-based access control, and security hardening (CSRF, rate limiting, token rotation).