Add llms.txt for AI content discovery

[nyechiel]

Summary

• Adds llms.txt as a dynamic Liquid template that auto-updates post counts and listings as new posts are published
• Categorizes posts by topic (AI, Career/Leadership, Networking/OpenStack) using tag-based filtering
• Adds Llms-txt: directive to robots.txt per the llms.txt specification

Test plan

• Verify llms.txt renders at /llms.txt with correct post count and categorized listings
• Verify robots.txt includes the Llms-txt directive
• Confirm localhost URLs are replaced with nyechiel.com in production build