cleanup: Remove development files before upstream PR

Remove temporary files created during development process:
- pymongo-async-tutorial.md: PyMongo learning reference
- pymongo-gridfs-async-tutorial.md: GridFS learning reference
- PROGRESS.md: Development progress tracking
- CLAUDE.md: AI development tool usage guide

These files were used during development but are not appropriate
for upstream contribution to the official MongoEngine repository.

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

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

Author: jeonghanjoo