docs: add runtime proof ledger

[escott-]

Summary

• publish a runtime proof ledger for Claude Code, Codex, and Goose
• distinguish Buzz/ACP startup support from a sourced signed ContextStream reply
• record the current Goose interoperability finding and its two tested provider paths
• add tests that prevent future startup-only claims from being labeled end-to-end verified

Test plan

• npm test
• node -e "JSON.parse(require('fs').readFileSync('compatibility.json','utf8'))"
• git diff --check main...HEAD