[codex] Add AI output evidence verifier

[newmattock]

Addresses #13

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

Fresh validation on 2026-05-17 21:13 PDT from a /tmp checkout of this PR:

• npm test -> ai-output-evidence-verifier tests passed
• npm run demo -> project not ready, recommendation hold_for_evidence_fix, 3 outputs, 5 claims, 3 sources, source coverage 3/3 known, 4 blockers, 2 warnings, 6 reviewer tasks
• npm run demo audit digest -> sha256:03c6970a21b3082dbbb20321da1cda4f56e19ff339a4defbba436b7084c3846b
• npm run demo:video -> renderer completed and wrote docs/demo.mp4
• file docs/demo.mp4 docs/demo.svg -> MP4 v2 and SVG image data
• git diff --check -> clean
• rg -n "(api[_-]?key|secret|token|password|private[_-]?key|wallet|0x[a-fA-F0-9]{40})" ai-output-evidence-verifier -> no matches

/claim #13