Add context and structure to project listings

[scottescue]

Summary

• add concise context to each public project and expose it through the project content schema
• explain how the public projects relate to recent private client work on the homepage and projects page
• redesign the projects index as a structured list with a split introductory section
• keep project-card title presentation consistent across semantic h2 and h3 heading levels

Impact

Visitors get clearer context for what each project demonstrates and how the public repositories relate to Scott’s recent production experience. The projects index is easier to scan while preserving the homepage card presentation and document heading hierarchy.

Validation

• npm run format
• npm run build (Astro check: 0 errors, 0 warnings, 0 hints; 7 routes built)
• Playwright visual review of the homepage and projects page at 1440px and 390px widths
• Lighthouse CI: 9 runs across /, /projects/, and /writing/, all scoring 100 for performance, accessibility, best practices, and SEO
• git diff --check