Bootstrap Registration Toggle

Add the ability to enable/disable user registration via an environment variable in a Phoenix 1.8 app generated with phx.gen.auth.

Prerequisites

Verify before starting:

1. Phoenix 1.8 app with phx.gen.auth completed
2. A registration controller exists (e.g., UserRegistrationController)
3. A login page exists with a "Sign up" link

App Name Detection

Detect the app module name from mix.exs (e.g., MyApp) and the OTP app name (e.g., my_app). Replace all template references accordingly.

Phase 0: Discovery

CRITICAL: Before generating anything, audit the target app to understand what's already in place.