chore: reorganize QA documentation files into docs/qa/ directory

Moved QA documentation from project root to organized docs/qa/ directory:
- Moved: EPIC_3_FIX_VALIDATION.md → docs/qa/EPIC_3_FIX_VALIDATION.md
- Removed: HANDOFF.md (old location, new version already in docs/qa/)

This improves project organization and keeps all QA/testing documentation
in a centralized location.

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

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

Author: salacoste