Migrate remaining /index URIs to /SKILL in universe/SKILL.md

[Berkkirik]

Summary

skills/universe/SKILL.md still uses the legacy /index suffix on roboflow:// resource URIs. PR #7 migrated this exact pattern (/index → /SKILL) in skills/product-navigation/features-by-page.md after the index.md → SKILL.md rename in 7db9842, but missed two siblings in universe/SKILL.md. This applies the same fix.

Before

## Related Skills

- `roboflow://skills/data-management/index` — managing datasets after import
- `roboflow://skills/training-and-evaluation/index` — training on forked data

After

## Related Skills

- `roboflow://skills/data-management/SKILL` — managing datasets after import
- `roboflow://skills/training-and-evaluation/SKILL` — training on forked data

Repro

grep -rn '/index' skills/ --include='*.md'
# skills/universe/SKILL.md:150:- `roboflow://skills/data-management/index` — managing datasets after import
# skills/universe/SKILL.md:151:- `roboflow://skills/training-and-evaluation/index` — training on forked data

After this PR: no matches.

Test plan

• grep -rn '/index' skills/ --include='*.md' returns no matches.
• Pattern matches the canonical form used in features-by-page.md:116 (roboflow://skills/plans-and-pricing/SKILL).
• Both referenced targets exist: skills/data-management/SKILL.md and skills/training-and-evaluation/SKILL.md.