chore: migrate tests from Jest to Vitest

[JoshuaKGoldberg]

Switches from the old Jest config, which extended kcd-scripts/jest, to Vitest.

This was a little more involved than I'd first thought (hence this PR coming a week later than I expected). At first I tried a manual switch-over but got into a confusing state where imports weren't being properly mocked out with vi.mock. I then restarted with Copilot and used Research-Plan-Implement. Copilot nailed it in one go, with just a little docs & scripts cleanup needed on my end (I manually reviewed every file).

The only major behavioral difference in tests is that nock is now used in src/repo/__tests__/index.js instead of internal mocking of modules. The new tests keep the functions as asynchronous, whereas the old ones switched them to synchronous for testing (weird!). I actually like this as more of an end-to-end / comprehensive test, rather than specifically looking at implementation details of what that file imports.

I think we can ignore the failing coverage indirect changes check. If you look at the indirect changes in Codecov they're mostly on comment lines.

[JimMadge]

Looks good. I think the loss of coverage through indirect changes is just the cost of switching test frameworks. We can start focusing on maintaining/increasing coverage after we have this sorted.