Add NuGet release pipeline (#9)

Added a release pipeline to automate NuGet package publishing. This
workflow handles versioning, building, testing, packaging, and
publishing to NuGet.org, as well as creating a GitHub Releases with
relevant artifacts.

Author: HaGGi13

.github/workflows/release.yaml

@@ -0,0 +1,112 @@
+name: Release to NuGet
+
+on:
+  push:
+    tags:
+      - 'v[0-9]+.[0-9]+.[0-9]+'   # e.g., v1.0.0, v2.3.1
+
+permissions:
+  contents: write # Needed for GitHub Release creation
+  id-token: write # Needed for GitHub OIDC token issuance
+
+env:
+  DOTNET_CLI_TELEMETRY_OPTOUT: 1
+  DOTNET_NOLOGO: 1
+  libraryPath: './src/DurationMancer/DurationMancer.csproj'
+  testsPath: './tests/DurationMancer.Tests/DurationMancer.Tests.csproj'
+  buildConfiguration: 'Release'
+
+jobs:
+  release:
+    name: Pack & Publish
+    runs-on: ubuntu-latest
+    environment: nuget # GitHub Environment with protection rules (recommended)
+    steps:
+
+      # --- Checkout ---
+      - name: Code checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      # --- GitVersion ---
+      - name: Install GitVersion
+        uses: gittools/actions/gitversion/setup@v4.2.0
+        with:
+          versionSpec: '6.5.x'
+      - name: Determine Version
+        id: version_step # step id used as reference for output values
+        uses: gittools/actions/gitversion/command@v4.2.0
+        with:
+          arguments: '/output buildserver /l console /config GitVersion.yaml /updateprojectfiles' # have to self-define all arguments; /updateprojectfiles not impl. in execute task
+
+      # --- .NET Setup ---
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: 10.0.x
+
+      # --- Restore ---
+      - name: Restore library
+        run: dotnet restore ${{ env.libraryPath }}
+      - name: Restore tests
+        run: dotnet restore ${{ env.testsPath }}
+
+      # --- Build ---
+      - name: Build library ${{ env.GitVersion_FullSemVer }}
+        run: >
+          dotnet build ${{ env.libraryPath }}
+          --no-restore
+          -c ${{ env.buildConfiguration }}
+      - name: Build tests
+        run: >
+          dotnet build ${{ env.testsPath }}
+          --no-restore
+          -c ${{ env.buildConfiguration }}
+
+      # --- Test (safety net before publish) ---
+      - name: Test
+        run: >
+          dotnet test ${{ env.testsPath }}
+          --no-build
+          -c ${{ env.buildConfiguration }}
+          --logger "GitHubActions;summary.includePassedTests=true;summary.includeSkippedTests=true"
+
+      # --- Pack ---
+      - name: Pack NuGet package
+        run: >
+          dotnet pack ${{ env.libraryPath }}
+          --no-build
+          -c ${{ env.buildConfiguration }}
+          -o ./nupkg
+          /p:PackageVersion=${{ env.GitVersion_FullSemVer }}
+
+      # --- Validate package ---
+      - name: Validate NuGet package
+        run: dotnet nuget verify ./nupkg/*.nupkg --all || true # 'verify' checks signatures; use 'true' fallback if unsigned
+
+      # --- Trusted Publishing: OIDC login
+      - name: NuGet login (OIDC → temp API key)
+        env:
+          NUGET_USER: ${{ secrets.NUGET_USER }}
+        uses: NuGet/login@v1
+        id: nuget_login
+        with:
+          user: $NUGET_USER
+
+      # --- Push to nuget.org ---
+      - name: Push to NuGet.org
+        env:
+          NUGET_API_KEY: ${{ steps.nuget_login.outputs.NUGET_API_KEY }}
+        run: >
+          dotnet nuget push ./nupkg/*.nupkg
+          --api-key $NUGET_API_KEY
+          --source https://api.nuget.org/v3/index.json
+          --skip-duplicate
+
+      # --- GitHub Release ---
+      - name: Create GitHub Release
+        uses: softprops/action-gh-release@v2
+        with:
+          generate_release_notes: true
+          files: ./nupkg/*.nupkg