Add SudachiPy Markdown vocabulary scripts

[ssaattww]

Summary

• add SudachiPy-based Markdown vocabulary extraction for .md and .txt files
• add a SudachiPy-based Markdown whitelist checker with existing focused modes
• document the new shared scripts in review-enforcer
• add a user-readable design doc at design/review-enforcer-markdown-whitelist-rebuild-design.md for whitelist rebuild behavior and review flow
• keep review-enforcer/SKILL.md focused on agent-executable rules, and remove the extra reference policy file

Closes #37

Validation

• python3 -m py_compile skills/review-enforcer/scripts/extract-markdown-vocabulary-sudachi.py skills/review-enforcer/scripts/check-markdown-whitelist-sudachi.py
• git diff --check
• Verified from IbisDuck with SudachiPy dependencies: vocabulary extraction, stdin focused check, textlint, cspell, and review r2