Skill Usage Metrics track which skills agents actually use

## Description
Track and visualize which skills from `.ai-team/skills/` are actually referenced in orchestration logs. Helps teams understand what expertise is being leveraged and identify gaps.

## Approach
- Parse orchestration logs for skill references
- Build frequency map: skill name usage count over time
- Add skills usage section to dashboard
- Consider chart showing skill usage trends

## Acceptance Criteria
- [ ] Parse orchestration logs for skill references
- [ ] Dashboard section showing skill usage frequency
- [ ] Time-based trend showing which skills are used most
- [ ] Identify skills that exist but are never referenced