Review README.md and SDK docs for public release

[devonartis]

Summary

Priority: HIGH — Before merging develop → main and going public, review all user-facing documentation for accuracy, broken links, stale references, and diagram correctness.

Checklist

Docs

• README.md — links, badges, quick start, demo section
• docs/getting-started.md
• docs/concepts.md
• docs/developer-guide.md
• docs/api-reference.md
• docs/testing-guide.md
• docs/sample-apps/ (01-08)
• docs/concepts-agent-cryptographic-identity.md
• CONTRIBUTING.md
• demo/BEGINNERS_GUIDE.md
• demo/PRESENTERS_GUIDE.md

Diagrams

• Verify all handdrawn diagrams are accurate
• Verify all Mermaid diagrams render correctly
• Verify all Excalidraw diagrams are current

Cleanup

• Remove any references to old v0.2.0 API, vendored broker, or internal paths
• Verify all links resolve (no 404s)

Ref: devonartis/agentwrit#31