The lint task was failing as one of the mappings also enabled linting the nested code blocks nested in markdown.
As I thought all the detected lint errors are irrelevant for demonstrating the plugin, I added them to the existing rule ignores.

I removed the code block language mapping from node to js as mentioned in the issue.
Should I add documentation for this case such as setting parserOptions: { sourceType: 'commonjs' }?

I think at this point we don't need to do anything to cater to CommonJS, specifically. I'm not sure what node is meant to indicate in Markdown anyway.

The lint task was failing as one of the mappings also enabled linting the nested code blocks nested in markdown. As I thought all the detected lint errors are irrelevant for demonstrating the plugin, I added them to the existing rule ignores.

Can you explain this a bit more? I'm not quite sure I follow what happened here. On HEAD linting passes as-is, so I don't just want to disable rules without understanding what changed.

One of the added mappings from code block language was from markdown to md.
This is also the case in the README.md section "What gets linted").. As such the processor is called recursively which was not the case before (just forgot to run the linter on all files).
I ignored the rules such as the console.log (no-console) or function without jsdoc (jsdoc/require-jsdoc) in the nested code blocks are for what I think are demonstration purposes.
The same reason as why no-var for markdown code block was disabled before this PR.