Skip the book where a tier has none, instead of failing

Skip the book where a tier has none, instead of failing A Component repository has a README and no book, so the shared check ran the builder against one and collected the refusal as a failure. --if-applicable makes a sweep skip quietly; asking for a book directly still gets the error, because there it is the answer. Co-Authored-By: Claude Opus 5 (1M context) Claude-Session: https://claude.ai/code/session_01XmzqahsmAXBn5wJab92jP2