Add Agent class architecture documentation

Add comprehensive documentation explaining the Agent class architecture and how it extends Durable Objects through PartyKit's Server class. This document demystifies the three-layer architecture (DurableObject → Server → Agent) and covers all major Agent features including:

- State management with this.state and this.setState()
- SQL template tag for database queries
- RPC and callable methods via WebSockets
- Built-in task queue with this.queue
- Scheduled execution with this.schedule
- MCP client integration
- Email handling capabilities
- Context management with AsyncLocalStorage
- Error handling and cleanup methods

Synced from cloudflare/agents PR #636

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

Author: github-actions[bot]