MacDown should automatically detect when the open file is changed and reload it

Nobody has claimed this yet.

If I open a file in MacDown but edit and save it elsewhere, MacDown does not immediately detect the change and reload the file. MacDown does have some logic to detect changes and revert to what's on disk, but I don't fully understand it.

The behavior I'm looking for is:

If the file open in MacDown is saved, then automatically detect changes to the file and reload it from disk without prompting.
If the file open in MacDown has unsaved changes, then automatically detect changes to the file and prompt the user for action.
In both cases MacDown should be able to immediately detect external changes to the file as soon as they happen. I don't know how this works, but I know that Visual Studio Code, for example, does this seamlessly.

Start by locating MacDown's existing logic for detecting external file changes and trace how an open document is reloaded or reverted. Verify the behavior for saved files and for files with unsaved changes, including the prompt path; done means external saves are detected immediately and each case follows the requested action.

Written by the indexing model from the issue text.