feat(artifacts): establish Aurora visual system

[jmagar]

Problem

Generated HTML artifacts were dense, visually inconsistent, and often harder to scan than their Markdown source. The PR review report also exposed cramped content, incomplete use of Aurora status colors, decorative navigation that did not provide useful movement, and template families that had drifted from one another.

Result

• establishes a shared Aurora artifact presentation system across reports, PR reports, research, proposals, specs, sessions, and docs
• adds full-bleed headers, responsive spacing, clearer typography and hierarchy, semantic status colors, icons, hover/focus states, and reduced-motion support
• replaces nonfunctional result filters with focused Overview, Findings, and Evidence navigation
• adds a dedicated evidence destination to the PR report template
• updates the artifact-generation skill and design guidance so future HTML artifacts must provide a clearer visual surface than equivalent Markdown
• adds contract tests covering the shared templates, navigation, accessibility behavior, responsive layout, and design tokens

Validation

• python3 -m unittest discover -s plugins/artifacts/tests -p 'test_*.py' — 23 passed
• plugins/scripts/check-all — plugin schema, 46 plugin tests, 11 artifact tests, marketplace alignment, and documentation checks passed
• rendered all seven template families at 1440px and 390px; every page matched the viewport width with no horizontal overflow
• isolated mergeability probe against current origin/main passed