Improve DX if ok creates a packages.yml

[haavardeide]

Context, see: https://oslokommune.slack.com/archives/CV9EGL9UG/p1736235504130289

When running ok pkg add ... and there is no packages.yml we generate one for the user, but: we should give a notice that we have done so, and add information that you must add DefaultPackagePathPrefix: boilerplate/github-actions to the file if you are installing github action boilerplates.

Other things considered:

• detect that we are inside a .github/workflows directory, but I don't think we should do that type of magic (yet)
• add a flag to the command: ok pkg add ... --type github-actions but that would require the user to know this flag on initial creation

But for now, since we are not doing this command very often, we should just give a good notice and see if that will nudge the user in the right direction.

[Kielo87]

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