Add comprehensive README documentation

[jbfromsd]

Resolves #3 — Adds a comprehensive README to the repository.

What's included

• Project description — Clear explanation of what fuel-agent-kit does
• Features list — All capabilities at a glance
• Installation instructions — npm install with peer dependency note
• Quick Start — Minimal example to get running
• Configuration reference — TypeScript interface with all options documented
• Supported Models — Table of OpenAI, Anthropic, and Gemini models
• Programmatic Usage — Direct function call examples for all operations (swap, transfer, supply, borrow, liquidity, balance)
• AI Agent Tools — Table of all tools available via natural language
• Environment Variables — Copy-paste ready env setup
• Development — Build, test, format, and lint commands

All documentation is derived directly from the source code (FuelAgent.ts, tools.ts, agent.ts, package.json).