API key limited to chosen sandboxes, approved in the browser

An agent with no key can now ask for a key that reaches only the sandboxes it names, whichever key or connector made them: runtime keys create --sandbox <id> --actions get,exec. An owner or admin approves it from a link, seeing each sandbox and what the key may do. Without create it cannot start, wake or restart them; a running one is billed as usual until its own time limit. See asked for in the browser.