Trim prompt runtime context

[shinpr]

Summary

• Trim low-value explanatory prompt content while preserving operational contracts and final checks
• Remove deprecated fullstack recipes from the dev-workflows plugin export while keeping them in dev-workflows-fullstack
• Add scope discipline guidance and restore frontend QA mechanism discovery for design docs
• Bump package and plugin versions to 0.20.0

Validation

• npm run sync:check
• npm run check:skills-index
• git diff --check
• claude plugin validate --strict dev-workflows
• claude plugin validate --strict dev-workflows-frontend
• claude plugin validate --strict dev-workflows-fullstack
• claude plugin validate --strict dev-skills
• claude plugin validate --strict .claude-plugin/marketplace.json