feat(sdk): add implicit Microsoft.VSSDK.BuildTools package reference

- SDK now automatically includes VSSDK.BuildTools (version 17.* by default)
- Users can override version with VssdkBuildToolsVersion property
- Update README to remove explicit BuildTools references from examples

Author: CalvinAllen

README.md

@@ -105,7 +105,6 @@ Replace your entire `.csproj` file with the SDK-style format:
 
   <ItemGroup>
     <PackageReference Include="Microsoft.VisualStudio.SDK" Version="17.*" />
-    <PackageReference Include="Microsoft.VSSDK.BuildTools" Version="17.*" PrivateAssets="all" />
   </ItemGroup>
 
 </Project>
@@ -138,11 +137,12 @@ Convert from `packages.config` to `PackageReference` format in your `.csproj`:
 ```xml
 <ItemGroup>
   <PackageReference Include="Microsoft.VisualStudio.SDK" Version="17.*" />
-  <PackageReference Include="Microsoft.VSSDK.BuildTools" Version="17.*" PrivateAssets="all" />
   <!-- Add other packages your extension uses -->
 </ItemGroup>
 ```
 
+> **Note:** `Microsoft.VSSDK.BuildTools` is automatically included by the SDK.
+
 #### Step 6: Handle VSCT Files
 
 If you have `.vsct` files, they're automatically included. Remove any explicit `<VSCTCompile>` items unless you need custom metadata.
@@ -182,7 +182,6 @@ If you prefer to set up manually, create a `.csproj` file:
 
   <ItemGroup>
     <PackageReference Include="Microsoft.VisualStudio.SDK" Version="17.*" />
-    <PackageReference Include="Microsoft.VSSDK.BuildTools" Version="17.*" PrivateAssets="all" />
   </ItemGroup>
 
 </Project>

src/CodingWithCalvin.VsixSdk/Sdk/Sdk.Vsix.props

@@ -14,6 +14,18 @@
               Condition="Exists('$(MSBuildThisFileDirectory)..\analyzers\dotnet\cs\CodingWithCalvin.VsixSdk.Generators.dll')" />
   </ItemGroup>
 
+  <!--
+    Implicit package references for VSIX development.
+    Users can override the version by setting VssdkBuildToolsVersion before SDK import.
+  -->
+  <PropertyGroup>
+    <VssdkBuildToolsVersion Condition="'$(VssdkBuildToolsVersion)' == ''">17.*</VssdkBuildToolsVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.VSSDK.BuildTools" Version="$(VssdkBuildToolsVersion)" PrivateAssets="all" />
+  </ItemGroup>
+
   <PropertyGroup>
     <!-- Identify that this SDK is in use -->
     <UsingCodingWithCalvinVsixSdk>true</UsingCodingWithCalvinVsixSdk>