refactor(io): migrate nest file extension from .zip to .opnest

Add FileExtension and FileFilter constants to NestFormat and update all
references across Console, MCP, Training, and WinForms projects.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.IO/NestFormat.cs

@@ -5,6 +5,9 @@ namespace OpenNest.IO
 {
     public static class NestFormat
     {
+        public const string FileExtension = ".opnest";
+        public const string FileFilter = "Nest Files (*.opnest)|*.opnest";
+
         public static readonly JsonSerializerOptions JsonOptions = new()
         {
             PropertyNamingPolicy = JsonNamingPolicy.CamelCase,