Engineering-grade code development with modular architecture, clear interfaces, and professional project structure. Use this skill when: (1) Starting any new coding project, (2) Building features that may grow in complexity, (3) User requests maintainable/scalable/professional code, (4) Projects expected to span multiple conversation turns, (5) Code that needs to be readable, testable, or reusable. This skill ensures code is modular (small focused files), consistent (predictable patterns), and navigable (clear project structure) - enabling efficient long-context development.