Register GitHub OAuth App for Copilot integration

Summary

The Copilot panel currently uses the VS Code GitHub OAuth Client ID (Iv1.b507a08c87ecfe98) for authentication during development and testing. Before the Copilot feature ships to users, we need to determine whether this is acceptable long-term or whether we need our own.

Action Items

1. Determine if we can use the VS Code Client ID — Check with the VS Code team or OSPO whether other Microsoft products are permitted to use this Client ID for Copilot authentication, even temporarily. If allowed, document the agreement and any constraints (e.g., scope, rate limits, branding).
2. If not permitted — Register our own GitHub OAuth App via OSPO at repos.opensource.microsoft.com and replace the Client ID constant in src/components/copilot-panel/src/Auth/CopilotAuthManager.ts (line 29). Update any related OAuth redirect URIs or scopes as needed.

Context

Self-service GitHub OAuth App registration is available through the microsoft GitHub org admin via OSPO. This is a pre-release blocker — it must be resolved before the Copilot feature ships, but does not block continued development on feature/copilot.