Improve GitHub discovery metadata

[intraq-dev-ai]

Summary

• Strengthen README H1 and opening paragraph for GitHub/Google discovery
• Add missing topic guidance including llm, rag, and ai-dashboard-builder
• Add GitHub discovery checklist and social preview SVG source
• Include prior public repository positioning/examples/license/dashboard AI changes from the branch

Validation

• git diff --check
• public secret/client scan over changed discovery files
• npm test
• npm run build in Node 24 Docker environment