[codex] Add AI output evidence verifier

[newmattock]

Summary

• adds ai-output-evidence-verifier, a dependency-free evidence gate for AI-assisted research tool outputs
• verifies source coverage, missing source IDs, retracted/stale sources, high-risk unsupported claims, required AI disclosures, reviewer tasks, and deterministic audit digest generation
• includes synthetic sample data, tests, requirement mapping, SVG walkthrough, and a generated MP4 demo

Validation

• npm test
• npm run demo
• npm run demo:video
• git diff --check
• rg -n "OPENAI|ANTHROPIC|API[_-]?KEY|SECRET|TOKEN|PASSWORD|PRIVATE KEY|system prompt|developer message" ai-output-evidence-verifier || true
• file docs/demo.mp4 -> ISO Media, MP4 v2 [ISO 14496-14]

/claim #13