feat: add grant escrow smart contract

[Dharmesh070294]

Summary

Added the core GrantStream escrow smart contract implementation using Hardhat and Solidity.

Changes

• Added GrantStreamEscrow.sol
• Added USDC-based grant funding flow
• Added milestone submission with IPFS evidence URI
• Added verifier approval and rejection flow
• Added automatic USDC release on milestone approval
• Added Base Sepolia deploy configuration
• Added deployment script
• Added MockUSDC for local testing
• Added Hardhat tests for the core escrow flow

Testing

Ran:

npm run compile
npm test

7 passing

[vercel]

Someone is attempting to deploy a commit to the tee's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Dharmesh070294]

The smart contract implementation is ready for review.

Local verification completed:

npm run compile
npm test

Result: 7 passing tests.