ci: dump msbuild log to artifact so we can debug failures via API

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

.gitea/workflows/build.yml

@@ -80,7 +80,26 @@ jobs:
         # `msbuild /restore` runs restore and build in the same MSBuild process,
         # so the package cache + project.assets.json + assembly resolution stay
         # consistent (avoids NETSDK1064).
-        run: msbuild OutlookAddin.sln /restore /t:Build /p:Configuration=Release /p:Platform="Any CPU" /m
+        # /flp dumps detailed log to build.log so we can retrieve it via the
+        # Gitea Actions artifacts API even when MSBuild fails mid-target.
+        shell: powershell
+        run: |
+          msbuild OutlookAddin.sln /restore /t:Build /p:Configuration=Release /p:Platform="Any CPU" /m `
+            /flp:logfile=build.log`;verbosity=detailed `
+            /flp1:logfile=build-errors.log`;errorsonly `
+            /flp2:logfile=build-warnings.log`;warningsonly
+          if ($LASTEXITCODE -ne 0) { exit $LASTEXITCODE }
+
+      - name: Upload build logs
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: build-logs-${{ github.run_id }}
+          path: |
+            build.log
+            build-errors.log
+            build-warnings.log
+          retention-days: 7
 
       - name: Test
         run: dotnet test src/OutlookAddin.Tests/OutlookAddin.Tests.csproj --configuration Release --no-build --logger "trx;LogFileName=test-results.trx"