fix(prefabs): wire missing C# handler for editor/prefab-stage resource

The mcpforunity://editor/prefab-stage resource was wired Python-side
in PR #1013 ("Prefab stages integration") via
Server/src/services/resources/prefab_stage.py, but its dispatched
`get_prefab_stage` command name was never given a C# handler.
Reading the resource hits CommandRegistry's not-found throw:

    Unknown or unsupported command type: get_prefab_stage

This change adds GetPrefabStage at
MCPForUnity/Editor/Resources/Editor/GetPrefabStage.cs, returning the
5 fields the Python PrefabStageResponse pydantic model expects
(isOpen, assetPath, prefabRootName, mode, isDirty) via
PrefabStageUtility.GetCurrentPrefabStage(). Mirrors the EditorState.cs
sibling resource's shape.

No Python-side or schema changes required.

Author: slikk66

MCPForUnity/Editor/Resources/Editor/GetPrefabStage.cs

@@ -0,0 +1,43 @@
+using System;
+using MCPForUnity.Editor.Helpers;
+using Newtonsoft.Json.Linq;
+using UnityEditor.SceneManagement;
+
+namespace MCPForUnity.Editor.Resources.Editor
+{
+    /// <summary>
+    /// Returns information about the currently open Prefab Stage (Isolation or
+    /// In-Context mode), or { isOpen = false } when no prefab is being edited.
+    ///
+    /// Wires up the existing <c>mcpforunity://editor/prefab-stage</c> resource
+    /// (Server/src/services/resources/prefab_stage.py) which dispatches the
+    /// <c>get_prefab_stage</c> command name expected by this handler.
+    /// </summary>
+    [McpForUnityResource("get_prefab_stage")]
+    public static class GetPrefabStage
+    {
+        public static object HandleCommand(JObject @params)
+        {
+            try
+            {
+                var stage = PrefabStageUtility.GetCurrentPrefabStage();
+                if (stage == null)
+                    return new SuccessResponse("No prefab stage open.", new { isOpen = false });
+
+                var root = stage.prefabContentsRoot;
+                return new SuccessResponse("Retrieved prefab stage info.", new
+                {
+                    isOpen = true,
+                    assetPath = stage.assetPath,
+                    prefabRootName = root != null ? root.name : null,
+                    mode = stage.mode.ToString(),
+                    isDirty = stage.scene.isDirty,
+                });
+            }
+            catch (Exception e)
+            {
+                return new ErrorResponse($"Error getting prefab stage: {e.Message}");
+            }
+        }
+    }
+}