What does cs-feat-impl do?
Feature process stage 2 – proceed step-by-step according to paradigm-dimension steps defined in the design section of the {slug}-checklist.yaml. Each step decides its own files to modify and reports results uniformly. Triggered when the user says "方案确认了开始实现", "按方案写代码", or "开工". Prerequisite: design is approved and checklist exists. If encountering cases beyond the design, return to discuss the plan instead of pushing through.