Add Agent class architecture documentation

Syncs documentation from cloudflare/agents PR #636 which adds comprehensive explanations of the Agent class architecture, including its inheritance chain from DurableObject → Server → Agent.

This document provides:
- Detailed explanation of the three-layer architecture
- Understanding of underlying Durable Object primitives
- PartyKit Server layer enhancements
- Agent-specific features like state management, scheduling, queuing, MCP integration, and email handling
- Code examples for all major Agent APIs

Source: cloudflare/agents#636

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Co-Authored-By: Claude <[email protected]>

Author: github-actions[bot]