[Arch] Document Core Package Role in 3-Way CLI Pattern

[jordanpartridge]

Overview

Document and implement guidelines for github-client as the core package in the 3-way CLI deployment pattern.

Tasks

• Document readonly class design decisions
• Add integration examples with CLI packages
• Update testing guidelines for resource mocking
• Document DTO transformation patterns
• Add cross-package integration tests

Current Challenges

• Testing readonly resources in CLI packages
• DTO transformation for CLI output
• Environment agnostic design