GitHub Radius: Application Graph Proposal

[zachcasper]

This pull request adds a new design note documenting the architectural challenges and execution model for integrating GitHub Radius with GitHub. The document outlines how the architecture shifts from a centralized control plane to a multi-client model with a Git-based data store, describes the proposed data and application graph models, and details the user journey for deploying applications using GitHub Copilot and GitHub Actions.

Key additions include:

Architecture and Execution Model:

• Introduced a new execution model for GitHub Radius, shifting from a centralized control plane to a multi-client architecture using a JSON-based graph database stored in a Git orphaned branch as the authoritative data store.
• Described how synchronous operations are handled by a Radius agent within GitHub Copilot, while asynchronous deployments are managed by the Radius control plane via GitHub Actions.

Data and Application Graph Model:

• Defined a unified data model for the application graph, supporting three views: application model, planned application graph, and deployed application graph, each with different levels of resource detail.

User Journey:

• Outlined a step-by-step user journey for deploying applications, including modeling, environment setup, resource planning, deployment, and real-time visualization of application state.# Description