Use verification on save instead of on change

This mode tends to be more stable, and gives the steps narrative more control over when you observe the verification results.

Should be configurable on the containers as a VS Code setting, but the steps also need to be edited to tell the user to save occasionally.

Start by locating the container configuration and the workshop steps that describe verification. Configure verification to run on save through a VS Code setting, and update the steps to tell users to save periodically; done means both the setting and the narrative reflect this workflow.

Written by the indexing model from the issue text.