Scope analyzer PackageReferences to SDK-style projects only in Directory.Build.props

[Copilot]

Directory.Build.props was injecting analyzer PackageReference items into all projects globally, including legacy non-SDK .csproj files in examples/Net4.8 that use packages.config, breaking restore behavior for those projects.

Description

Added Condition="'$(UsingMicrosoftNETSdk)' == 'true'" to both ItemGroup blocks so analyzer references are only applied to SDK-style projects:

<ItemGroup Condition="'$(UsingMicrosoftNETSdk)' == 'true'">
  <AdditionalFiles Include="$(MSBuildThisFileDirectory)BannedSymbols.txt" />
</ItemGroup>

<ItemGroup Condition="'$(UsingMicrosoftNETSdk)' == 'true'">
  <PackageReference Include="Roslynator.Analyzers" ... />
  <!-- remaining analyzer references -->
</ItemGroup>

Legacy packages.config projects (e.g., examples/Net4.8/**) are unaffected.

Type of change

• Bug fix

How Has This Been Tested?

• Verified Directory.Build.props no longer injects PackageReference items into non-SDK projects

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

Additional context

Addresses feedback from #58.

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[Copilot]

Pull request overview

Scopes globally injected analyzer PackageReference items in Directory.Build.props so they only apply to SDK-style projects, preventing restore issues for legacy packages.config projects under examples/Net4.8.

Changes:

• Added Condition="'$(UsingMicrosoftNETSdk)' == 'true'" to the AdditionalFiles ItemGroup.
• Added the same condition to the analyzer PackageReference ItemGroup.

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