[Docs] Class diagram (Mermaid) for Core/Agent/AI/Web

[JasonEran]

Epic: #8

Goal

Provide UML-style class diagrams for the project (course requirement).

Acceptance Criteria

• Mermaid diagram committed under docs/diagrams.
• Includes key classes and relationships across services.

[JasonEran]

Final Evidence

Completed by PR #51:

• [v2.3][Process] Complete templates + Mermaid class diagrams #51
• Merge commit: 3ea5247

Acceptance mapping:

• Mermaid class diagram committed under docs/diagrams/Class-Diagram-v2.3.md.
• Covers key classes/relationships for Core, Agent, AI, and Web.