"Guide for building Rails applications following 37signals/DHH patterns. Use when developing Rails apps for: (1) Controller design and routing, (2) Model concerns and business logic, (3) Authentication without Devise, (4) Hotwire/Turbo/Stimulus patterns, (5) CSS architecture, (6) Testing with Minitest. Emphasizes vanilla Rails, rich models, thin controllers, CRUD-everything, and database-backed infrastructure over Redis."