Sandbox disk persistence and clearer capacity messages

Sandbox disk retention across stop/start is verified in production, alongside clearer capacity and GitHub permission messages.

• Files in /workspace, home directories, installed tools, settings and saved AI sign-in files survive stop/start within the sandbox lifetime. Production checks confirmed disk retention after a fresh boot. Running processes and open sessions end at a stop. The lifetime continues while stopped, and expiry or deletion removes the sandbox's temporary files.
• Busy sandbox sizes are refused immediately with a clear capacity message instead of silently waiting. Available sizes still depend on regional host capacity, and a paid plan does not guarantee an immediate start.
• GitHub connections missing required runner permissions show a warning and a link to correct access. Affected jobs are refused instead of remaining in the queue. Personal GitHub account support and custom runner profiles remain switched off pending activation.