A workflow whose only trigger is `workflow_call` never runs on its own,
so its sidebar entry always showed an empty run list and a "Disable
workflow" action that has no effect.
Following the discussion with @bircni and @Zettat123, these workflows
are now listed in a separate collapsed "Reusable workflows" group. Their
page shows "This workflow can only be called by other workflows."
instead of "no runs", and "Disable workflow" and "Create status badge"
are hidden. A workflow disabled before it became `workflow_call`-only
keeps its label and "Enable workflow". "Other workflows" again only
lists workflows whose file is gone.
Fixes: https://github.com/go-gitea/gitea/issues/39385
Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
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