refactor(abstractions): rename ILogger to IPluginLogger, doc Initialize

Addresses code quality review of ed1c2d0:
- ILogger would collide with Microsoft.Extensions.Logging.ILogger and Serilog.ILogger
  in any plugin file that imports both namespaces; renamed to IPluginLogger
- IAcDreamPlugin.Initialize now has an XML doc clarifying its lifecycle contract

src/AcDream.Plugin.Abstractions/IPluginLogger.cs

@@ -0,0 +1,9 @@
+// src/AcDream.Plugin.Abstractions/IPluginLogger.cs
+namespace AcDream.Plugin.Abstractions;
+
+public interface IPluginLogger
+{
+    void Info(string message);
+    void Warn(string message);
+    void Error(string message, Exception? exception = null);
+}