ci: build.yml + release.yml 보강 (서명 옵션 + workflow_dispatch)

- build.yml 추가: PR/push 시 솔루션 빌드 + MSIX 산출 검증, 14일 보관
- release.yml: PFX_BASE64 / PFX_PASSWORD secrets 있으면 자동 서명, 없으면
  unsigned. workflow_dispatch로 수동 트리거도 가능.
- 릴리즈 본문에 인증서 등록 + 사이드로드 가이드 inline 포함.

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

.github/workflows/build.yml

@@ -0,0 +1,43 @@
+name: Build
+
+on:
+  push:
+    branches: [master, main]
+  pull_request:
+    branches: [master, main]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: windows-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: 9.0.x
+
+      - name: Setup MSBuild
+        uses: microsoft/setup-msbuild@v2
+
+      - name: Restore + Build solution
+        shell: pwsh
+        run: |
+          dotnet build EverythingToJpeg.slnx -c Release --nologo
+
+      - name: Build MSIX
+        shell: pwsh
+        run: |
+          ./packaging/GenerateAssets.ps1
+          ./packaging/BuildMsix.ps1 -Configuration Release -Platform x64
+
+      - name: Upload MSIX artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: EverythingToJpeg-x64-msix-${{ github.sha }}
+          path: packaging/dist/EverythingToJpeg-x64.msix
+          retention-days: 14