Add new sandbox flow, repo expansion, and VSCode
Refactor backend exposeVscode: allow unauthenticated queries for public
sandboxes (isNull userId) and defer AT Protocol agent creation until a
sandbox has an at:// uri.
Add GitLab repo expansion (CLI + web hook), a useExpandRepo hook and a
/new page to create sandboxes from repo URLs. Add client/mutation and UI
button to expose a VS Code preview and open the returned preview URL.