Add `security-engineer` skill

[niravpatel27]

Summary

Add a security-engineer skill that guides secure coding practices — identifying vulnerabilities, reviewing auth/authz, and suggesting fixes for common security issues.

What this skill should do

• Review code for OWASP Top 10 vulnerabilities
• Guide on secure authentication and authorization patterns
• Flag insecure handling of secrets, tokens, and credentials
• Suggest input validation and output encoding patterns
• Help write security-focused tests

Reference

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

Acceptance Criteria

• security-engineer/SKILL.md exists with correct frontmatter
• Skill identifies at least one real vulnerability in a sample code snippet