ci: vcxproj를 packages.config 형식으로 복귀, BuildMsix가 nuget.exe 자동 처리

증상: VS 2022 Enterprise(GitHub windows-latest)에서 PackageReference 형식
vcxproj 빌드 시 ResolveNuGetPackageAssets가 "Sequence contains no elements"
오류로 실패. native vcxproj + PackageReference + msbuild restore 조합의
알려진 회귀.

해결:
- src/EverythingToJpeg.Shell/EverythingToJpeg.Shell.vcxproj: PackageReference
  제거, packages.config + Microsoft.Windows.ImplementationLibrary.targets
  imports 복귀, EnsureNuGetPackageBuildImports target 복귀.
- src/EverythingToJpeg.Shell/packages.config 재추가.
- packaging/BuildMsix.ps1: nuget.exe 없으면 dist.nuget.org에서 자동 다운로드,
  packages.config 기반 restore를 packages/ 폴더에 수행.
- .gitignore: packages/, tools/ 제외.

이러면 사용자 PC(VS 2026)와 CI(VS 2022) 모두 동일한 흐름으로 빌드.

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

packaging/BuildMsix.ps1

@@ -75,8 +75,25 @@ if ($LASTEXITCODE -ne 0) { throw 'dotnet publish 실패' }
 # ---- 2) C++ Shell DLL ----
 Write-Host ''
 Write-Host '[2/5] C++ Shell DLL 빌드'
-& $msbuild $shellProj /t:Restore /p:RestorePackagesConfig=true /p:Configuration=$Configuration /p:Platform=$Platform /v:minimal | Out-Host
-if ($LASTEXITCODE -ne 0) { throw 'Shell restore 실패' }
+
+# nuget.exe 자동 다운로드 (없으면)
+$nuget = Get-Command nuget.exe -ErrorAction SilentlyContinue
+if (-not $nuget) {
+    $nugetExe = Join-Path $repoRoot 'tools\nuget.exe'
+    if (-not (Test-Path $nugetExe)) {
+        New-Item -ItemType Directory -Path (Split-Path $nugetExe) -Force | Out-Null
+        Write-Host '  nuget.exe 다운로드 중…'
+        Invoke-WebRequest -Uri 'https://dist.nuget.org/win-x86-commandline/latest/nuget.exe' -OutFile $nugetExe
+    }
+    $nugetCmd = $nugetExe
+} else {
+    $nugetCmd = $nuget.Source
+}
+
+$packagesDir = Join-Path $repoRoot 'packages'
+& $nugetCmd restore (Join-Path $repoRoot 'src\EverythingToJpeg.Shell\packages.config') -PackagesDirectory $packagesDir | Out-Host
+if ($LASTEXITCODE -ne 0) { throw 'NuGet restore 실패' }
+
 & $msbuild $shellProj /p:Configuration=$Configuration /p:Platform=$Platform /m /v:minimal | Out-Host
 if ($LASTEXITCODE -ne 0) { throw 'Shell build 실패' }
 $shellDll = Join-Path $repoRoot ("src\EverythingToJpeg.Shell\$Platform\$Configuration\EverythingToJpeg.Shell.dll")