Improve SEO, structured data, and LLM discoverability

[nyechiel]

Summary

• Add llms-full.txt with complete post content (145 KB) so LLMs can ingest the entire blog in one fetch - this is the biggest lever for fixing the "ChatGPT thinks the blog stopped in 2022" problem
• Fix Twitter/X card metadata: twitter:site was rendering as @ (empty) and twitter:creator as @Nir Yechiel (space in handle) - both now correctly show @nyechiel
• Add supplementary JSON-LD on blog posts with publisher and author.sameAs social links (Twitter, LinkedIn, GitHub) - neither of which jekyll-seo-tag generates on its own
• Override minima's footer.html to handle author as an object (needed after changing author from a string to an object with name/url/twitter in _config.yml)
• Reference llms-full.txt in robots.txt

Test plan

• Verify footer displays "Nir Yechiel" (not a raw hash)
• Verify Twitter/X icon still appears in footer social links
• Verify link previews show correct metadata when sharing a post URL (Slack, LinkedIn, etc.)
• Visit /llms-full.txt and confirm all 31 posts appear with full content
• Check a blog post page source for two valid JSON-LD blocks (one from jekyll-seo-tag, one supplementary with publisher/sameAs)
• Check a non-post page (e.g., /about/) has only one JSON-LD block (no supplementary)