refactor(slides): Renumber week 2 lectures (5-8) to account for X-hour

- lecture4.md -> lecture5.md (Data Cleaning & Preprocessing)
- lecture5.md -> lecture6.md (Tokenization)
- NEW lecture7.md (X-Hour: Text Classification Workshop)
- lecture6.md -> lecture8.md (POS Tagging & Sentiment Analysis)

Updated all internal lecture number references and "Next lecture" pointers.
Removed generated files (.html, .pdf, .tex) which will be regenerated.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: jeremymanning