weichen
02198f1ceb
Why: the prompt system's design rationale (layering, the eight module patterns, trigger semantics, override chain) needs to live with the docs so future prompt work follows it instead of re-deriving it. Evidence: guide content distilled from the implemented library and the Claude prompt catalog analysis recorded in the task card. Impact: new docs/guides/prompt-library-design.md; no code change. Refs: feature/prompt-library task card.