Removes the three separate PRD skills (create, edit, validate) in favor of the
unified bmad-prd skill. Updates module-help.csv, PM agent menu, workflow map,
getting-started tutorial, commands reference, customize/help SKILL.md examples,
and the website workflow-map diagram. Adds Recipe 6 (Advanced Integration
Patterns) to expand-bmad-for-your-org.md covering external_sources,
external_handoffs, doc_standards, and swappable templates.