attribution cleanup

[muraalee]

1. Summary hero section — redesigned with donut rings for AI/Human percentages, large bold numbers for Total Lines and Prompt Efficiency, plus a stacked split bar with legend
2. Metric renamed — "Commit:Prompt" ratio (1:1.5) → "Prompt Efficiency" (commits/prompts as 0.67 decimal)
3. Trend charts — each chart now in its own GitHub Box with Box-header/Box-body, horizontal + vertical SVG grid lines
4. By Tool / Top Models — using GitHub Box with Box-header + Box-row classes for native styling
5. All colors updated — mid-tone colors that work in both light and dark themes (#E07B53, #57AB5A, #A371F7, #C69026, #8b949e)
6. Pure Primer CSS — removed all custom CSS classes, using GitHub's native .Box, .Box-header, .Box-body, .Box-row, .Box-title classes