Implement Architecture Recommendations for OneMount

[bcherrington]

Description

Implement architecture recommendations to improve the overall structure and organization of the OneMount project.

Rationale

The current project structure has some inconsistencies and doesn't fully align with Go community best practices. Implementing these recommendations will improve code organization, maintainability, and testability.

Impact

This implementation will affect the overall project structure and will improve code organization, maintainability, and testability.

Relevant Documentation

• Test Implementation Execution Plan

Implementation Notes

• Introduce internal/ for private packages and pkg/ for public libraries
• Align with community practices for Go project structure
• Break down large main.go routines into discrete services (e.g., AuthService, FilesystemService)
• Improve readability and testability of the codebase
• Define interfaces for external dependencies (Graph API, DB)
• Inject implementations for easier mocking in tests