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.yaml

@@ -14,7 +14,9 @@ icon_url: https://gitea.hellion-forge.cloud/Hellion-Forge/PluginNameTemplate/raw
 assembly_version: 0.1.0
 testing_assembly_version: 0.1.0
 
-dalamud_api_level: 13
+# Dalamud.NET.Sdk 15.0.0 targets API level 15. Check the SDK release notes
+# when bumping (the API level usually moves with major SDK updates).
+dalamud_api_level: 15
 
 categories:
   - utility
@@ -24,7 +26,7 @@ tags:
 
 changelog: |
   v0.1.0
-  - Initial release.
+  - Initial release based on the official Dalamud sample plugin (goat demo intact).
 
-# Don't override DalamudPackager defaults (Dalamud.NET.Sdk 13+ handles icon and
-# image_urls automatically via the .csproj <None Include="images\icon.png" />).
+# Don't override DalamudPackager defaults. The SDK reads icon and image_urls
+# from the .csproj <None Include="images\icon.png" /> entry automatically.