ci: drop upload-artifact step from build.yml

actions/upload-artifact@v7 fails on Gitea Actions — the GitHub
artifact API has compatibility gaps the Gitea runtime layer does not
fully cover, and v7 specifically tripped exitcode 1 on the Strato
runner. The build itself runs fine; the artefact was never consumed
by anything (release.yml does its own latest.zip lookup), so the
cleanest fix is to make build.yml a pure compile-health check
without artefact upload.

.github/workflows/build.yml

@@ -51,11 +51,3 @@ jobs:
 
       - name: Build (Release)
         run: dotnet build HellionChat/HellionChat.csproj --configuration Release --no-restore
-
-      - name: Upload build output
-        uses: actions/upload-artifact@v7
-        with:
-          name: HellionChat-build-${{ github.run_number }}
-          path: HellionChat/bin/Release/**/HellionChat/**
-          if-no-files-found: warn
-          retention-days: 14