docs: rewrite README for the current USDC bounty workflow

[notjamyfriench]

Summary

This updates the README to match the current product behavior and bounty flow.

What changed

• Reframed project as a GitHub issue bounty marketplace paid in USDC
• Added maintainer + solver lifecycle and claim steps
• Documented webhook setup required for automated status transitions
• Added API endpoint overview for bounties/github/user routes
• Added accurate local setup and environment variable section
• Removed outdated NFT/SOL positioning and corrupted text artifacts

Closes #40

[vercel]

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

A member of the Team first needs to authorize it.

[notjamyfriench]

@andr-drgm quick follow-up: this README rewrite is aligned with the current codebase and payout flow (USDC bounty lifecycle, webhook setup, API routes, env setup). If you want, I can adapt wording to your preferred tone in a fast patch.

[notjamyfriench]

Quick update: I pushed a focused README improvement commit that adds an explicit issue #40 acceptance checklist for faster review mapping. Commit: 9550ca2 on this PR branch. If you want, I can also split the checklist into a dedicated section near Contributing for even faster scanability.