closes #19603

Summary

• Adds XamlStyler as a local dotnet tool for consistent XAML formatting across src/SamplesApp/
• Formats all ~1,480 existing SamplesApp XAML files with the new configuration
• Adds a GitHub Actions CI check that fails PRs with unformatted XAML (passive mode)
• Updates contributor documentation (code style guide, SamplesApp guide, add-sample skill, AGENTS.md)

Configuration highlights (src/SamplesApp/Settings.XamlStyler)

• Tab indentation (matches .editorconfig)
• First attribute stays on same line, continuations indented by 4 spaces
• Attribute reordering enabled (x:Class → xmlns → x:Key/Name → layout → rest)
• Safe defaults: no child reordering, design-time references preserved

For contributors

dotnet tool restore                    # one-time after clone
dotnet xstyler -d src/SamplesApp -r   # format all SamplesApp XAML
dotnet xstyler -f path/to/file.xaml   # format a single file

Test plan

• dotnet tool restore succeeds
• dotnet xstyler -d src/SamplesApp -r processes 1,480 files without errors
• dotnet xstyler -d src/SamplesApp -r -p passes (idempotent check)
• CI workflow triggers on PRs touching SamplesApp XAML
• SamplesApp builds and runs correctly after formatting

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