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.sln

@@ -0,0 +1,21 @@
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.0.31903.59
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "PluginNameTemplate", "PluginNameTemplate.csproj", "{B12C5E91-7A3D-4E8F-A2C1-9D4E5F6A7B8C}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{B12C5E91-7A3D-4E8F-A2C1-9D4E5F6A7B8C}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{B12C5E91-7A3D-4E8F-A2C1-9D4E5F6A7B8C}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{B12C5E91-7A3D-4E8F-A2C1-9D4E5F6A7B8C}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{B12C5E91-7A3D-4E8F-A2C1-9D4E5F6A7B8C}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+EndGlobal