docs: update AGENTS.md and README.md

AGENTS.md:
- Added Rule 27: mobile-first responsive design
- Added Section 21: Responsive Design Rules
  - Breakpoint reference table
  - Layout rules per section
  - Typography scaling across breakpoints
  - What hides or shows at different breakpoints
  - Testing checklist (6 widths to verify)

README.md:
- Updated Next.js 14 to Next.js 15
- Added React 19 to tech stack

Author: onerandomdevv