Consolidate migration documentation

- Merge 8 separate docs into 1 comprehensive MIGRATION_GUIDE.md
- Move docs to a4d-python/docs/migration/ (better organization)
- Update CLAUDE.md for Python project (moved to a4d-python/docs/)
- Remove scattered docs from root directory

What's included in MIGRATION_GUIDE.md:
- Strategy & architectural decisions
- Technology stack (Astral toolchain)
- Architecture (per-tracker, BigQuery state)
- Key migration patterns (R → Python)
- Phase-by-phase checklist
- Code examples for critical components
- Success criteria

Single source of truth for the migration, easier to maintain.

Author: pmayd