docs: add technical guide for agent extensibility

[D3n3bX]

Description

This PR introduces the comprehensive guide for expanding Perry's capabilities by implementing new specialized agents.

Technical Requirements

• BaseAgent Inheritance: New agents must inherit from to benefit from standardized LLM initialization and observability.
• Async Reasoning Flow: The documentation explains how to implement the method as an to yield objects for the UI reasoning block.
• Modular Factory Registration: Details the process for adding agents to using local imports to avoid circular dependency issues.
• Semantic Routing Configuration: Explains the two-step process of adding intents to and activating new routes in the service.

Architecture Benefits

This guide ensures that any new module (e.g., Marketing, HR, or Logistics) maintains the same structural integrity, professional persona, and full traceability provided by the core engine.