v0.1.8 - release limits documentation

Maintenance evidence release for Codex for OSS review.\n\n- Added README known limits for current offline claim verification, optional/fallback integrations, adversarial held-out measurement status, and demo CORS.\n- Added release check commands before tagging.\n- Added hosted MCP smoke guidance for deployed /mcp URLs.\n- Completes the public-doc acceptance criteria for issue #16.\n- CI and CodeQL are green on this commit; open code-scanning alerts remain 0.