core: move guide prompt, schema, user message and validator into @plannotator/core (#1605)

Moves GUIDE_REVIEW_PROMPT, GUIDE_SCHEMA_JSON, buildGuideUserMessage and
validateGuideOutput verbatim from the private @plannotator/server into a new
browser-safe @plannotator/core/guide-prompt subpath, with their pure
dependencies (@plannotator/core/review-prompt, @plannotator/core/diff-type).
Server, shared and Pi re-export the same names from the same paths.