fix(site): show negative history deltas

[Seungpyo1007]

Summary

• show negative dataset history deltas instead of treating decreases as a baseline
• show category-level increases and decreases in the history timeline
• put History before Playground in the nav and hero CTA flow
• stop PR metadata automation from adding persistent tracker issue references that can auto-close long-running issues

Tracking

• Closes Track homepage and site improvements #19

Verification

• npm.cmd run build
• git diff --check