Split by year into years/{2023,2024,2025,2026}.md and shrink main README

GitHub's 512KB README auto-render limit was truncating the overview page, so viewers on github.com/dair-ai/AI-Papers-of-the-Week could not scroll past the early sections. Move the 170 weekly entries into four per-year files and keep the main README as a compact index:

- years/2023.md - 312KB, 52 weekly entries
- years/2024.md - 275KB, 52 weekly entries
- years/2025.md - 755KB, 52 weekly entries (largest year)
- years/2026.md - 203KB, 14 weekly entries so far

Main README is now 20KB with the intro, subscribe link, and a year-grouped TOC whose anchors point to the corresponding year file (e.g. years/2026.md#top-ai-papers-of-the-week-april-6---april-12---2026). Each year file also links back to the main index. All 170 TOC anchors were verified to resolve to a real section heading.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>