refactor(lecture17): consolidate RAG slides and replace OpenAI with free FLAN-T5

- Reduce from 22 slides to 19 through strategic merging
- Add flow diagram for RAG pipeline: Query → Embed → Retrieve → Augment → Generate
- Replace OpenAI API with free HuggingFace FLAN-T5 (no API key required)
- Merge redundant slides: knowledge problem, embeddings, chunking, vector databases
- Update "Up next" to match actual lecture 18 title (BERT deep dive)
- Update assignment 4 submodule to simplified version with provided data

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jeremymanning