Merge official Dalamud sample plugin into template

- Replace Plugin.cs/ConfigWindow.cs skeleton with working sample code
- Add MainWindow.cs (goat-image demo + PlayerState/Lumina queries)
- Rename src/PluginConfiguration.cs → src/Configuration.cs (sample naming)
- Add Data/goat.png sample asset
- Add src/packages.lock.json (NuGet lockfile from sample)
- Add PluginNameTemplate.sln solution file
- Bump csproj from Dalamud.NET.Sdk 13.0.0 → 15.0.0
- Bump yaml dalamud_api_level: 13 → 15
- Update README with sample-removal walkthrough and SDK-bump section

Template now builds end-to-end out of the box. Goat demo
intact for verification; strip per README when implementing
the real plugin.

PluginNameTemplate.csproj

@@ -1,8 +1,7 @@
-<Project Sdk="Dalamud.NET.Sdk/13.0.0">
+<Project Sdk="Dalamud.NET.Sdk/15.0.0">
   <!--
-    Replace `Dalamud.NET.Sdk/13.0.0` with the SDK version current at the time
-    you start your plugin. Check https://github.com/goatcorp/Dalamud.NET.Sdk
-    for the latest tag.
+    SDK 15.0.0 is current as of 2026-05. Check https://github.com/goatcorp/Dalamud.NET.Sdk
+    for the latest tag when you bump this template.
   -->
 
   <PropertyGroup>
@@ -20,12 +19,19 @@
 
   <ItemGroup>
     <!--
-      Add additional NuGet refs here as needed. Dalamud.NET.Sdk pulls in
-      Dalamud, ImGui.NET, FFXIVClientStructs, Lumina, etc. by default.
+      Sample asset bundled with the working demo (the goat-image referenced
+      by MainWindow.cs). Drop your real plugin assets here and adjust the
+      include-path. CopyToOutputDirectory ensures the file lands next to the
+      compiled DLL so PluginInterface.AssemblyLocation finds it.
     -->
+    <Content Include="Data\goat.png">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+      <Visible>false</Visible>
+    </Content>
   </ItemGroup>
 
   <ItemGroup>
-    <None Include="images\icon.png" Pack="true" PackagePath="\" />
+    <!-- Plugin icon used by the Dalamud manifest. Drop your real icon at images/icon.png. -->
+    <None Include="images\icon.png" Pack="true" PackagePath="\" Condition="Exists('images\icon.png')" />
   </ItemGroup>
 </Project>