docs: mark project as maintenance mode

[iiwish]

Summary

• mark the project as archived and in maintenance mode
• direct new integrations to ACP or official agent SDKs
• retain release history and fresh-evidence requirements
• keep the release-evidence self-test deterministic after its fixture ages past 24 hours

Validation

• git diff --check
• targeted real compatibility evidence verifier contract test
• npm run package:docs:check