fix(ci): flag prerelease tags as GitHub pre-releases (#4)

softprops/action-gh-release doesn't infer pre-release status from the tag, so
v7.0.0-preview.1 was published as a full release. Set prerelease from whether the
tag name contains a SemVer prerelease suffix ('-'), so preview tags are flagged
and stable tags (v7.0.0) remain full releases.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: ChrisonSimtian

.github/workflows/publish.yml

@@ -73,3 +73,6 @@ jobs:
         with:
           files: artifacts/packages/*.nupkg
           generate_release_notes: true
+          # Flag preview/prerelease tags (any tag with a SemVer prerelease suffix,
+          # e.g. v7.0.0-preview.1) as a GitHub pre-release; stable tags (v7.0.0) stay full releases.
+          prerelease: ${{ contains(github.ref_name, '-') }}