Add CopilotSkipCliDownload option to .NET SDK (#494)

Add MSBuild property CopilotSkipCliDownload that when set to true skips
automatic download and bundling of the Copilot CLI binary. This is useful
for users who install the CLI separately (e.g., via npm or a package manager)
and want to avoid the build-time download.

When skipped, users must ensure COPILOT_CLI_PATH or the CliPath option
points to a valid CLI installation at runtime.

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

Author: SteveSandersonMS

dotnet/src/build/GitHub.Copilot.SDK.targets

@@ -55,8 +55,8 @@
     <CopilotCliDownloadTimeout Condition="'$(CopilotCliDownloadTimeout)' == ''">600</CopilotCliDownloadTimeout>
   </PropertyGroup>
 
-  <!-- Download and extract CLI binary -->
-  <Target Name="_DownloadCopilotCli" BeforeTargets="BeforeBuild" Condition="'$(_CopilotPlatform)' != ''">
+  <!-- Download and extract CLI binary. Set CopilotSkipCliDownload=true to skip if you install the CLI separately. -->
+  <Target Name="_DownloadCopilotCli" BeforeTargets="BeforeBuild" Condition="'$(CopilotSkipCliDownload)' != 'true' And '$(_CopilotPlatform)' != ''">
     <Error Condition="'$(CopilotCliVersion)' == ''" Text="CopilotCliVersion is not set. The GitHub.Copilot.SDK.props file may be missing from the NuGet package." />
 
     <!-- Compute paths using version (now available) -->
@@ -90,7 +90,7 @@
   </Target>
 
   <!-- Copy CLI binary to output runtimes folder and register for transitive copy -->
-  <Target Name="_CopyCopilotCliToOutput" AfterTargets="Build" DependsOnTargets="_DownloadCopilotCli" Condition="'$(_CopilotPlatform)' != ''">
+  <Target Name="_CopyCopilotCliToOutput" AfterTargets="Build" DependsOnTargets="_DownloadCopilotCli" Condition="'$(CopilotSkipCliDownload)' != 'true' And '$(_CopilotPlatform)' != ''">
     <PropertyGroup>
       <_CopilotCacheDir>$(IntermediateOutputPath)copilot-cli\$(CopilotCliVersion)\$(_CopilotPlatform)</_CopilotCacheDir>
       <_CopilotCliBinaryPath>$(_CopilotCacheDir)\$(_CopilotBinary)</_CopilotCliBinaryPath>
@@ -101,7 +101,7 @@
   </Target>
 
   <!-- Register CLI binary as content so it flows through project references -->
-  <Target Name="_RegisterCopilotCliForCopy" BeforeTargets="GetCopyToOutputDirectoryItems" DependsOnTargets="_DownloadCopilotCli" Condition="'$(_CopilotPlatform)' != ''">
+  <Target Name="_RegisterCopilotCliForCopy" BeforeTargets="GetCopyToOutputDirectoryItems" DependsOnTargets="_DownloadCopilotCli" Condition="'$(CopilotSkipCliDownload)' != 'true' And '$(_CopilotPlatform)' != ''">
     <PropertyGroup>
       <_CopilotCacheDir>$(IntermediateOutputPath)copilot-cli\$(CopilotCliVersion)\$(_CopilotPlatform)</_CopilotCacheDir>
       <_CopilotCliBinaryPath>$(_CopilotCacheDir)\$(_CopilotBinary)</_CopilotCliBinaryPath>