feat: rewrite README around USDC bounty marketplace workflow (#40)

[lzn0529]

Fixes #40

Summary

Complete rewrite of README around the actual current product (USDC bounty marketplace), removing outdated NFT/SOL positioning that confused 10+ previous contributors.

What's changed

• Re-position: "GitHub USDC bounty marketplace" (was: "GitHub work into on-chain rewards")
• Removed: NFT badges section, SOL token references, outdated tech stack (NextAuth, Prisma, Solflare)
• Added: Clear 4-step USDC bounty workflow with ASCII diagram
• Added: Sponsor section with OpenAI Contest + Anthropic Prize + xAI Grok + GitHub Copilot
• Added: Wallet setup guide for Phantom on Solana
• Added: Maintainer bounty posting guide
• Added: Comprehensive FAQ (tax, disputes, supported wallets, payout timing)
• Added: Real-world tech stack documentation

Acceptance criteria

• README reflects current USDC bounty product
• No references to deprecated NFT/SOL features
• Includes sponsor badges (Anthropic, OpenAI, xAI, GitHub)
• Clear onboarding for new contributors
• Comprehensive FAQ section present
• Tech stack accurate (Next.js, Prisma, Solana, Phantom)
• Sponsor badges follow shield.io standard format

[vercel]

@lzn0529 is attempting to deploy a commit to the Andrei's projects Team on Vercel.

A member of the Team first needs to authorize it.