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>

src/EverythingToJpeg.Shell/EverythingToJpeg.Shell.vcxproj

@@ -99,12 +99,22 @@
   </ItemGroup>
 
   <ItemGroup>
+    <None Include="packages.config" />
     <None Include="Source.def" />
   </ItemGroup>
 
-  <ItemGroup>
-    <PackageReference Include="Microsoft.Windows.ImplementationLibrary" Version="1.0.260126.7" />
-  </ItemGroup>
-
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
+
+  <ImportGroup Label="ExtensionTargets">
+    <Import Project="..\..\packages\Microsoft.Windows.ImplementationLibrary.1.0.260126.7\build\native\Microsoft.Windows.ImplementationLibrary.targets"
+            Condition="Exists('..\..\packages\Microsoft.Windows.ImplementationLibrary.1.0.260126.7\build\native\Microsoft.Windows.ImplementationLibrary.targets')" />
+  </ImportGroup>
+
+  <Target Name="EnsureNuGetPackageBuildImports" BeforeTargets="PrepareForBuild">
+    <PropertyGroup>
+      <ErrorText>WIL NuGet 패키지가 복원되지 않았습니다. nuget restore를 먼저 실행하세요.</ErrorText>
+    </PropertyGroup>
+    <Error Condition="!Exists('..\..\packages\Microsoft.Windows.ImplementationLibrary.1.0.260126.7\build\native\Microsoft.Windows.ImplementationLibrary.targets')"
+           Text="$(ErrorText)" />
+  </Target>
 </Project>

src/EverythingToJpeg.Shell/packages.config

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<packages>
+  <package id="Microsoft.Windows.ImplementationLibrary" version="1.0.260126.7" targetFramework="native" />
+</packages>