content(slides): rewrite lectures 18-27 to box-first style

Rewrite all remaining lectures (L18-L27) to match the box-first
style established in L15-L17. All content now uses semantic boxes
(definition-box, note-box, tip-box, etc.), sentence case titles,
standardized frontmatter, and consistent Questions slides.

- L18-L20 (Week 6): BERT deep dive, variants, encoder applications
- L21-L23 (Week 7): GPT architecture, scaling to GPT-3, implementing GPT
- L24 (Week 9): Written from scratch as Agents & Tool Use (replaced
  duplicate RAG content with ReAct, Toolformer, function calling)
- L25-L26 (Week 9): MoE & efficiency, ethics/bias/safety
- L27 (Week 10): Final presentations with reflective closing

All lectures compiled to HTML and PDF.

Author: jeremymanning