Document multi-agent system features from PR #675

Add comprehensive documentation for the new Agency and AgentSystem features:

- Concepts: Multi-agent systems architecture and patterns
- API Reference: AgentSystem class with fluent configuration API
- API Reference: Agency Durable Object for agent coordination
- Guide: Step-by-step tutorial for building multi-agent systems

Key features documented:
- Tag-based tool and middleware selection
- Agent blueprint system for defining agent types
- Agency coordination for managing multiple agents
- Dynamic blueprint override at runtime
- Subagent spawning for hierarchical agent systems

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: github-actions[bot]