use mock-fs to isolate fs access

https://www.npmjs.com/package/mock-fs

We just mock the fs before the action function is run, and re-set after. Not sure if there will be concurrency issues

The issue names the action function but no files or tests, so first locate that entry point and review how filesystem access is exercised. Read the mock-fs documentation, then determine whether mocking before the action and resetting afterward is safe with concurrent operations; done means isolated filesystem access without concurrency regressions.

Written by the indexing model from the issue text.