[codex] Add homepage post and category lists

[Ridealist]

Summary

• Add reusable homepage includes for recent posts and category links.
• Render 5 recent posts with title, date, category, and excerpt.
• Render homepage category links from _data/navigation.yml sidebar-category with post counts.
• Wire the new includes into the homepage and add compact row/grid styles scoped to homepage.css.

Validation

• bundle exec jekyll build
• git diff --check HEAD~2..HEAD
• Verified local homepage renders 5 recent post rows and 10 category links.
• Verified Fine-tuning LLMs, 우아한테크코스, 1 post, and 15 posts appear in the local homepage response.
• Verified existing Sass deprecation warnings from Minimal Mistakes remain unchanged.

Closes #10