Add agent visibility: llms.txt + HTML fallback + JSON-LD

[hrhrng]

Summary

• Add /llms.txt and /llms-full.txt following the llmstxt.org standard — AI agents can fetch these to understand the site and its URL API
• Add #agent-info HTML block in index.html — visible to WebFetch/crawlers, auto-hidden when React mounts via #root:not(:empty) ~ #agent-info
• Add JSON-LD structured data and meta description for search engines
• Add <link rel="help"> pointing to /llms.txt for discoverability

Three layers of agent visibility

Layer	Path	Purpose
llms.txt	/llms.txt	Standard LLM site docs — concise overview + URL API
llms-full.txt	/llms-full.txt	Full documentation: all view modes, examples, skill install
HTML fallback	index.html #agent-info	SPA fallback — WebFetch sees content directly in HTML

Test plan

• npm run build succeeds, llms.txt and llms-full.txt present in dist/
• Visit site in browser — React app loads normally, #agent-info is hidden
• Fetch /llms.txt directly — returns readable Markdown with URL API docs
• Disable JS in browser — #agent-info content is visible