ci: drop broken REPO_TOKEN, grant contents:write instead (#117)

The release asset upload was hitting 'Bad credentials' because
secrets.REPO_TOKEN is expired / not set. Remove the explicit token
override and add job-level contents:write so the default GITHUB_TOKEN
has enough scope to create the GitHub release. NuGet push is unaffected
and already worked via NUGET_API_KEY.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erwan-joly

.github/workflows/dotnet.yml

@@ -11,6 +11,8 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
     steps:
     - uses: actions/checkout@v5
     - name: Setup .NET
@@ -47,5 +49,4 @@ jobs:
       uses: softprops/action-gh-release@v2
       with:
         files: ${{ steps.build_artifact.outputs.ARTIFACT_PATH }}
-        token: ${{ secrets.REPO_TOKEN }}