Reduce docs agent context

[AvdLee]

Summary

• reduce docs/AGENTS.md to commands, safety and quality gates, concise conventions, and task routing
• move feature/frontmatter schemas, content examples, local-serving safety, and blog voice/SEO/attribution requirements into docs/CONTENT-AUTHORING.md
• update docs/CLAUDE.md to route content work to the scoped guide
• align blog guidance with the current local MDX content model

Context metrics

• before: 10,814 bytes / 224 lines
• after: 2,006 bytes / 52 lines
• reduction: 8,808 bytes (81.5%) / 172 lines (76.8%)

Validation

• relative Markdown links validated
• npx prettier --check AGENTS.md CONTENT-AUTHORING.md CLAUDE.md
• npm run lint
• npm run format:check
• npm run typecheck
• npm run build
• npm run knip