Add AI skills slides, refine Section 3 flow, fix notes panel restore

- Add 3 AI skills slides: why, ideal Ruby structure (with diagram), limitations
- Simplify slide 34 to "For AI doc consumption: evaluated, nothing proven yet"
- Hide "Better tools for writing docs" slide (recap, not future work)
- Fix notes panel disappearing after save (was stripped but never restored)
- Add section labels to all Section 2 slides

Author: st0012