docs: add transfer readiness checklist

[chubes4]

Summary

• Add a transfer-readiness architecture checklist covering package boundaries, runtime artifact lifecycle, extension seams, browser runtime dependencies, ability contracts, and security gates.
• Link the checklist from the root README and architecture overview so incoming maintainers can find the transfer review surface.
• Identify existing follow-up issues and create transfer-specific cleanup issues for package namespace migration and browser/runtime dependency classification.

Follow-up issues

• Architecture cleanup: deduplicate shared object, hash, and artifact manifest helpers #344
• Architecture docs: explain Codebox package boundaries and runtime artifact lifecycle #346
• Support caller-provided sandbox MU plugins and task invocation for repair runtimes #317
• Generate runtime reference index for browser artifacts #316
• Transfer cleanup: replace personal package namespace and repo URLs #357
• Transfer audit: classify browser runtime and packaging dependencies #358

Closes #355.

Verification

• npm run build
• npm run package-distribution-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the transfer-readiness documentation, created follow-up issues, and ran verification. Chris remains responsible for review and merge.