Add `data-privacy-engineer` skill

[niravpatel27]

Summary

Add a data-privacy-engineer skill that helps handle personal data responsibly — covering GDPR/CCPA compliance, data minimization, consent flows, and PII handling.

What this skill should do

• Identify PII and sensitive data in code and schemas
• Guide on data minimization — only collect what's needed
• Help implement consent flows that meet GDPR/CCPA requirements
• Review data retention and deletion policies
• Flag risky data handling patterns

Reference

See existing skills like lfx-backend-builder/SKILL.md for pattern and format.
Each skill lives in its own directory with a SKILL.md file and markdown instructions.

Acceptance Criteria

• data-privacy-engineer/SKILL.md exists with correct frontmatter
• Skill gives useful privacy guidance on a sample feature with user data