Fix all three CI build failures

- Android/Linux: Patch Markdown.Avalonia.props for Linux CI (submodule
  only defines PackageTargetFrameworks for Windows_NT, causing empty
  framework identifier error on ubuntu-latest)
- Windows MSIX: Fix null plugins crash in MsixTools (update submodule),
  add plugins: [] to msix.yml for explicit empty config
- Remove global.json (was pinning SDK version unnecessarily for CI)
- Add .gitattributes to enforce LF line endings for shell scripts

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sharpninja

.gitattributes

@@ -0,0 +1 @@
+*.sh text eol=lf

.github/workflows/build-android.yml

@@ -65,6 +65,17 @@ jobs:
         with:
           dotnet-version: '9.0.x'
 
+      - name: Patch Markdown.Avalonia for Linux
+        run: |
+          # The submodule props only defines PackageTargetFrameworks for Windows_NT.
+          # Add a Linux fallback so NuGet restore can resolve the project graph.
+          sed -i '/<\/Project>/i \
+          <PropertyGroup Condition=" '\''$(OS)'\'' != '\''Windows_NT'\'' ">\
+            <PackageTargetFrameworks>netstandard2.0<\/PackageTargetFrameworks>\
+            <DemoAppTargetFrameworks>netstandard2.0<\/DemoAppTargetFrameworks>\
+            <TestTargetFrameworks>netstandard2.0<\/TestTargetFrameworks>\
+          <\/PropertyGroup>' lib/Markdown.Avalonia/Markdown.Avalonia.props
+
       - run: dotnet workload install android
 
       - name: Decode keystore
@@ -166,6 +177,15 @@ jobs:
         with:
           dotnet-version: '9.0.x'
 
+      - name: Patch Markdown.Avalonia for Linux
+        run: |
+          sed -i '/<\/Project>/i \
+          <PropertyGroup Condition=" '\''$(OS)'\'' != '\''Windows_NT'\'' ">\
+            <PackageTargetFrameworks>netstandard2.0<\/PackageTargetFrameworks>\
+            <DemoAppTargetFrameworks>netstandard2.0<\/DemoAppTargetFrameworks>\
+            <TestTargetFrameworks>netstandard2.0<\/TestTargetFrameworks>\
+          <\/PropertyGroup>' lib/Markdown.Avalonia/Markdown.Avalonia.props
+
       - name: Build DEB package
         run: |
           chmod +x scripts/build-deb.sh

global.json

@@ -15,6 +15,8 @@ build:
   rid: win-x64
   selfContained: true
 
+plugins: []
+
 output:
   dir: artifacts