Structured workflow for adding any new feature to an existing project. Creates a features/<slug>/ directory containing a living spec (README.md), technical plan (plan.md), implementation log (changes.md), and review report (review.md). Covers requirements clarification, codebase pattern survey, plan approval gate, implementation, UML.md update, and diff review. Use whenever a user asks to add, build, or implement a new feature, integration, job, endpoint, or module.