shellcheck: add yaml helper script

[deer-wmde]

Since shellcheck does not parse the embedded shell scripts we have in our k8s job yaml files, I tried to see if they can be reliably extracted via yq, in order to lint them. This PR contains a script that extracts them in a git-ignored directory. I tried running this in CI on this repo fork here: https://github.com/deer-wmde/debug-wbaas-deploy/actions/runs/25557616246/job/75020526364 (sadly the output in this example run is bloated by all the other recommendations for "real" scripts, too)

weak points:

• the script assumes the currently used pattern of using shell -c command
• there might be more embedded shell scripts somewhere else in the repo, outside of /k8s/jobs/ (now or in the future)

Still I think this could be handy to have, as errors in these scripts are especially tricky.

https://phabricator.wikimedia.org/T302394

Bug: T302394

[deer-wmde]

Today I tried if this would work with shfmt too - it does in theory, but in practice would make them even more unreadable while looking at the YAML

This means currently the CI workflow #2740 would kinda need a messy order to run:

• shellcheck & shfmt
• then this script
• shellcheck again