Agent Skills: improve-codebase-architecture

Review a codebase for architectural friction, rank deep-module refactoring opportunities, and draft RFCs for safer interfaces and boundary-test strategies. Use when the user wants to improve architecture, identify refactoring seams, consolidate tightly coupled modules, deepen shallow modules, redesign an interface around a core concept, replace brittle unit tests with boundary tests, or turn an architecture review into an RFC or issue draft.