[CI/CD] Ephemeral PR preview environments with Hardhat fork

[nanaf6203-bit]

Problem Statement

PRs are validated against shared testnets. There's no per-PR isolated environment for full-stack integration.

Acceptance Criteria

• A "preview environment" workflow uses Vercel previews for the frontend and a Hardhat fork node for the backend
• GitHub PR bot posts the URL automatically
• Environment destroyed when PR closes

Implementation Notes

Vercel webhook → GitHub Action → ephemeral fork. Use wagmi http://preview-fork:8545.

Files Likely Affected

• .github/workflows/preview.yml
• src/config/wagmi.ts
• docs/preview.md

Difficulty / Effort

• Difficulty: Hard
• Effort: T-Shirt XL

Labels

ci-cd, priority:low, area:infrastructure, area:web3