Transition GitHub and Microsoft authentication to host-owned auth

### Summary

Transition GitHub and Microsoft authentication used by agents to the host-owned authentication model proposed in [microsoft/agent-host-protocol#404](https://github.com/microsoft/agent-host-protocol/pull/404).

### Current architecture

VS Code owns the authentication state and obtains credentials through its authentication providers, then brokers tokens to the agent host. This couples agent authentication to the VS Code client and extension host lifecycle.

### Desired architecture

- The agent host/runtime owns its GitHub and Microsoft account state and credentials.
- VS Code no longer retains authentication state solely to pass tokens into the agent host.
- When an interactive authentication flow is required, the host delegates client-specific UI operations such as opening a browser to VS Code through the Agent Host Protocol.
- Authentication state and flows follow the host-owned auth proposal while preserving an appropriate client-brokered path for sandboxed hosts.

### Scope

Focus initially on GitHub and Microsoft authentication, including account discovery, sign-in, sign-out, token refresh, and migration from the existing VS Code-owned token flow.