Add CLAUDE.md documentation file

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+MagicSpend is an ERC-4337 compliant paymaster smart contract that allows onchain accounts to present valid Withdraw Requests and receive funds. It's part of Coinbase's broader MagicSpend product enabling seamless asset usage onchain.
+
+The core contract (`src/MagicSpend.sol`) implements:
+- ERC-4337 paymaster functionality (EntryPoint v0.6 compatible)
+- Withdraw request validation with EIP-191 compliant signatures
+- Gas payment capabilities for smart contract wallets
+- Fund transfers during execution
+
+## Development Commands
+
+This is a Foundry-based Solidity project. Common development commands:
+
+### Testing
+```bash
+# Run all tests
+forge test
+
+# Run specific test file
+forge test --match-path test/MagicSpend.t.sol
+
+# Run tests with gas reporting
+forge test --gas-report
+
+# Run echidna fuzzing tests
+make echidna-test
+```
+
+### Building and Deployment
+```bash
+# Build contracts
+forge build
+
+# Deploy (requires .env file with RPC_URL and PRIVATE_KEY)
+make deploy
+```
+
+## Architecture
+
+### Core Contract Structure
+- `src/MagicSpend.sol` - Main contract implementing IPaymaster and Ownable
+- Key struct: `WithdrawRequest` containing signature, asset, amount, nonce, and expiry
+- Uses Solady libraries for optimized operations (SignatureCheckerLib, SafeTransferLib)
+- Integrates with account-abstraction interfaces
+
+### Test Organization
+- `test/` contains comprehensive unit tests for each contract function
+- `test/PaymasterMagicSpendBase.sol` - Base test contract with common setup
+- `test/echidna/` contains fuzzing tests using Echidna
+- Tests cover three main flows: gas payment only, fund transfer only, and combined
+
+### Dependencies
+- **Solady** - Gas-optimized library for auth, signatures, and transfers
+- **OpenZeppelin Contracts** - Security utilities
+- **Account Abstraction** - ERC-4337 interfaces and UserOperation types
+- **Smart Wallet** - Coinbase's smart contract wallet implementation
+- **Forge Std** - Foundry testing framework
+
+### Key Configuration
+- `foundry.toml` - Optimized for 999999 runs, custom remappings for dependencies
+- `echidna-config.yaml` - Configuration for property-based fuzzing tests
+- Maximum withdraw denominator prevents excessive withdrawals affecting paymaster reputation
+
+### Development Notes
+- Contract uses Solidity 0.8.23
+- Heavy focus on gas optimization (999999 optimizer runs)
+- Comprehensive test coverage including integration and property-based tests
+- EIP-191 signature validation for withdraw requests
+- Nonce tracking prevents replay attacks