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.

src/Plugin.cs

@@ -1,28 +1,85 @@
+using Dalamud.Game.Command;
 using Dalamud.IoC;
 using Dalamud.Plugin;
+using System.IO;
+using Dalamud.Interface.Windowing;
 using Dalamud.Plugin.Services;
+using PluginNameTemplate.Windows;
 
 namespace PluginNameTemplate;
 
 public sealed class Plugin : IDalamudPlugin
 {
-    [PluginService] public static IDalamudPluginInterface Pi { get; private set; } = null!;
-    [PluginService] public static IPluginLog Log { get; private set; } = null!;
-    [PluginService] public static ICommandManager Commands { get; private set; } = null!;
+    [PluginService] internal static IDalamudPluginInterface PluginInterface { get; private set; } = null!;
+    [PluginService] internal static ITextureProvider TextureProvider { get; private set; } = null!;
+    [PluginService] internal static ICommandManager CommandManager { get; private set; } = null!;
+    [PluginService] internal static IClientState ClientState { get; private set; } = null!;
+    [PluginService] internal static IPlayerState PlayerState { get; private set; } = null!;
+    [PluginService] internal static IDataManager DataManager { get; private set; } = null!;
+    [PluginService] internal static IPluginLog Log { get; private set; } = null!;
 
-    private readonly PluginConfiguration config;
+    private const string CommandName = "/pmycommand";
+
+    public Configuration Configuration { get; init; }
+
+    public readonly WindowSystem WindowSystem = new("PluginNameTemplate");
+    private ConfigWindow ConfigWindow { get; init; }
+    private MainWindow MainWindow { get; init; }
 
     public Plugin()
     {
-        this.config = PluginConfiguration.Load();
+        Configuration = PluginInterface.GetPluginConfig() as Configuration ?? new Configuration();
 
-        // Register your commands, hooks, windows, etc. here.
-        Log.Information("PluginNameTemplate loaded.");
+        // You might normally want to embed resources and load them from the manifest stream
+        var goatImagePath = Path.Combine(PluginInterface.AssemblyLocation.Directory?.FullName!, "goat.png");
+
+        ConfigWindow = new ConfigWindow(this);
+        MainWindow = new MainWindow(this, goatImagePath);
+
+        WindowSystem.AddWindow(ConfigWindow);
+        WindowSystem.AddWindow(MainWindow);
+
+        CommandManager.AddHandler(CommandName, new CommandInfo(OnCommand)
+        {
+            HelpMessage = "A useful message to display in /xlhelp"
+        });
+
+        // Tell the UI system that we want our windows to be drawn through the window system
+        PluginInterface.UiBuilder.Draw += WindowSystem.Draw;
+
+        // This adds a button to the plugin installer entry of this plugin which allows
+        // toggling the display status of the configuration ui
+        PluginInterface.UiBuilder.OpenConfigUi += ToggleConfigUi;
+
+        // Adds another button doing the same but for the main ui of the plugin
+        PluginInterface.UiBuilder.OpenMainUi += ToggleMainUi;
+
+        // Add a simple message to the log with level set to information
+        // Use /xllog to open the log window in-game
+        Log.Information($"===A cool log message from {PluginInterface.Manifest.Name}===");
     }
 
     public void Dispose()
     {
-        // Unregister anything that was registered above. Order matters —
-        // dispose UI before hooks, hooks before services.
+        // Unregister all actions to not leak anything during disposal of plugin
+        PluginInterface.UiBuilder.Draw -= WindowSystem.Draw;
+        PluginInterface.UiBuilder.OpenConfigUi -= ToggleConfigUi;
+        PluginInterface.UiBuilder.OpenMainUi -= ToggleMainUi;
+
+        WindowSystem.RemoveAllWindows();
+
+        ConfigWindow.Dispose();
+        MainWindow.Dispose();
+
+        CommandManager.RemoveHandler(CommandName);
     }
+
+    private void OnCommand(string command, string args)
+    {
+        // In response to the slash command, toggle the display status of our main ui
+        MainWindow.Toggle();
+    }
+
+    public void ToggleConfigUi() => ConfigWindow.Toggle();
+    public void ToggleMainUi() => MainWindow.Toggle();
 }