Add reputation sybil cluster guard

[Jorel97]

/claim #15

Summary

• Adds a distinct reputation-sybil-cluster-guard/ Community & User Reputation System slice for issue Community & User Reputation System #15.
• Audits synthetic reputation events for shared payout handles, recycled device fingerprints, burst endorsements, suspicious invitation chains, cross-project high-score review rings, risky reputation deltas, and missing quarantine controls.
• Emits deterministic JSON, Markdown, SVG, GIF, and MP4 demo artifacts from synthetic data.

Non-overlap

This is not another probation reinstatement, leaderboard eligibility/privacy, reputation review abuse, identity impersonation, badge renewal, credit-ledger reputation, completion reputation, or endorsement-ring-only slice. It focuses specifically on clustered sybil signals that can inflate reputation before rankings, bounty eligibility, or payout review.

Validation

• npm test -> passed
• npm run demo -> generated reports/sybil-cluster-report.json, reports/sybil-cluster-report.md, reports/summary.svg, and reports/demo-script.txt
• npm run demo:video -> generated reports/demo.gif and reports/demo.mp4
• git diff --check -> passed

Synthetic data only. No credentials, private user records, payment processors, wallets, social media accounts, live reputation services, or external APIs are used by this module.

AI-assisted with OpenAI Codex; I reviewed and locally verified the diff before submitting.