Add `ux-designer` skill

[niravpatel27]

Summary

Add a ux-designer skill that helps design user flows, wireframes, and UX copy — focusing on clarity, consistency, and accessibility.

What this skill should do

• Help map out user flows for a feature
• Guide writing clear, helpful UX copy (labels, errors, empty states)
• Identify usability issues in existing designs
• Suggest accessible design patterns
• Help prioritize UX improvements by user impact

Reference

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

Acceptance Criteria

• ux-designer/SKILL.md exists with correct frontmatter
• Skill gives useful output on a sample UX design task