Add `ai-engineer` skill

[niravpatel27]

Summary

Add an ai-engineer skill that guides building AI-powered features — covering prompt design, model integration, evals, and responsible AI patterns.

What this skill should do

• Help design and iterate on prompts
• Guide integrating LLM APIs (Anthropic, OpenAI, etc.)
• Suggest evaluation strategies for AI outputs
• Highlight responsible AI practices (bias, safety, transparency)
• Help debug unexpected model behavior

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

• ai-engineer/SKILL.md exists with correct frontmatter
• Skill gives useful guidance on a sample AI feature task