feat(blog): add BMAD requirements-to-tests presentation recap

[harrybin]

Summary

This PR adds a new blog post that recaps the BMAD presentation and the related GitHub Copilot demo workflow.

What is included

• New post: src/content/blog/bmad-anforderungen-tests-demo.md
• Complete required frontmatter for the blog collection
• English rewrite with improved readability
• Content based on research from:
  • presentation site (bmad-anforderungen-tests route)
  • demo branch bmad_all-steps-atdd_no-impl
  • exported Copilot chats in bmad-atdd-chat-histroy

Highlights in the article

• Explains the requirement-to-test gap shown in the presentation
• Describes the BMAD workflow used in the demo
• Walks through PRD, context generation, epics/stories, architecture update, TEA, traceability, and NFR checks
• Adds references to the presentation and demo resources

Validation

• npm run sync passed
• npm run build passed
• Build reported unrelated minor HTML width corrections in an existing generated post output