docs: add e2e tests refactoring plan

Add comprehensive analysis and refactoring roadmap for src/bin/e2e_tests.rs:

- Identify 21 specific improvement tasks across 3 phases
- Document current code smells and architecture issues
- Provide actionable roadmap with checkboxes for progress tracking
- Include proposed architecture with clear separation of concerns
- Add completion tracking section for collaborative development

This live documentation helps developers coordinate refactoring efforts
and track progress on improving the 735-line monolithic e2e test module.

Follows project naming convention using kebab-case for markdown files.

Author: josecelano