Full spec for ordered, sequentially unlocked, bundle-priced learning
paths, plus 16 clickable prototypes (public org site, learner LMS, and
teacher workspace) built on the packages/ui tokens and the minimal
org-landing theme. The prototypes are the UX source of truth; start at
prototypes/learning-paths/index.html. Also adds the write-prd skill
codifying the clarify -> write -> prototype workflow.
