As we use three dashes to separate sections, they can't be used for creating horizontal lines in the markdown. Markdown also allows you to use asterisks or underscores (https://www.markdownguide.org/basic-syntax/#horizontal-rules) but vscode likes to replace them with dashes when you save the file:
Before save:

After save:

Lets see if we can turn off this behavior just for markdown that is recognized as a prompt file.
While we're at it, probably ought to raise a warning if there are more than three --- lines in the file, as that probably indicates that the user is trying to include a horizontal line. Raise the warning on the third --- found if there are more than 3, and say "are you trying to include a horizontal rule? because we use --- to separate prompt sections, you won't be able to use it here. Try *** or ____ (3 underscores) instead."
As we use three dashes to separate sections, they can't be used for creating horizontal lines in the markdown. Markdown also allows you to use asterisks or underscores (https://www.markdownguide.org/basic-syntax/#horizontal-rules) but vscode likes to replace them with dashes when you save the file:
Before save:

After save:

Lets see if we can turn off this behavior just for markdown that is recognized as a prompt file.
While we're at it, probably ought to raise a warning if there are more than three
---lines in the file, as that probably indicates that the user is trying to include a horizontal line. Raise the warning on the third---found if there are more than 3, and say "are you trying to include a horizontal rule? because we use---to separate prompt sections, you won't be able to use it here. Try***or____(3 underscores) instead."