Misc usability improvements for sandbox CLI

[robinaugh]

This PR makes a few miscellaneous usability improvements to the sandbox CLI.

• Add more verbose --help documentation for rwx sandbox exec, to make LLMs better at using it - With these changes, I was able to have an LLM generate a test feature without wrestling with the sandbox tooling at all (and without creating an unnecessary script file).
• Check ready state before showing spinner, to prevent flickering the waiting message unnecessarily - The spinner gets printed over when using a terminal manually, so you don't even notice it, but Claude Code was showing it persistently, which made it look like it needed to wait for the sandbox to be ready in between each exec call.
• Use a specific run title format when creating a sandbox run - This makes them look different in the UI from a normal CLI run. I've currently not allowed this to be configurable, but we could.