Cloud compute errors say "sandbox" instead of workspace #437

The cloud compute service's errors call the workspace a "sandbox". docs/terms.md calls it the channel's workspace (a container in the pilot). These messages reach people through the app's "Cloud compute" refusals, so the wrong term shows up in the UI.

services/compute/index.ts:

• L89 The sandbox is stopped
• L114 Sandbox did not become ready in 120 seconds
• L191 Sandbox is already running; use resize to change its size
• L197 The sandbox was lost without a checkpoint; refusing to create an empty replacement
• L304 The sandbox is stopped; use start to restore it

Reword them in terms of the workspace and the app's actions. For example, "The cloud workspace is stopped; use Start compute to restore it." Rewording these messages doesn't change any stored format.

Found while writing up the cloud compute part of the automated study.