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.
