Simplify setup and clean up notebooks

- Add requirements.txt for notebook dependencies
- Simplify SETUP.md with clearer instructions
- Replace 01_working_memory_with_extraction_strategies with simpler 01_working_memory
- Update notebooks to use dotenv for environment variables
- Remove obsolete migration docs and fix scripts
- Add .gitignore for Python artifacts

Author: abrookins

python-recipes/context-engineering/.gitignore

@@ -0,0 +1,2 @@
+venv
+.env