Add security considerations document

Problem

A library handling keys and transactions should have explicit security documentation.

Recommendation

Create docs/security.md covering:

Key Management

• Never log private keys
• Use environment variables, not hardcoded
• Consider hardware wallets for production

RPC Security

• Don't use public RPCs for mainnet (rate limits, privacy)
• Use authenticated endpoints
• Consider running own node

Limit Configuration

• Start with conservative limits
• Adjust based on use case
• Monitor spending patterns

Smart Account Security

• Session key permissions
• Paymaster trust assumptions

Common Pitfalls

• ENS caching considerations
• Nonce management with concurrent txs
• Stablecoin depeg scenarios

Priority

Documentation - Security

Review the recommendations in the issue and create docs/security.md with sections for key management, RPC security, limit configuration, smart account security, and common pitfalls. Done means each listed security concern is documented clearly for users of the library; no tests are mentioned.