consider improving some TOCTOU subtleties

For example, if a user:
1. Runs `configlet sync -u --tests -e foo`
2. Waits at the prompt
3. Changes the `tests.toml` file for exercise `foo` in their track directory, e.g. adding `include = false` to a test.
4. Continues going through configlet prompts until completion

If I recall correctly, `configlet sync` still works, and ignores that the file was changed. We could document this, or consider producing a warning if the file was modified since read-time. But it's probably tricky (and not worth trying) to support arbitrary changes to a file that we're asking questions about.