chore: remove generated files from git

- Remove coverage.html (generated report file)
- Remove TESTING_NOTES.md (temporary development notes)
- Add coverage.html to .gitignore

Generated files and temporary notes should not be tracked in git.
Coverage reports change frequently and can be regenerated locally.

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

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

Author: jackspirou