fix: handle bare Assets/ path in manage_shader (#743)

Trim(/) was stripping the trailing slash before the StartsWith check,
so Assets/ became Assets which did not match Assets/. Now handles
the bare Assets case explicitly.

Co-authored-by: Claude Opus 4.6 <[email protected]>

Author: dsarno

MCPForUnity/Editor/Tools/ManageShader.cs

@@ -67,7 +67,11 @@ public static object HandleCommand(JObject @params)
             if (!string.IsNullOrEmpty(relativeDir))
             {
                 relativeDir = AssetPathUtility.NormalizeSeparators(relativeDir).Trim('/');
-                if (relativeDir.StartsWith("Assets/", StringComparison.OrdinalIgnoreCase))
+                if (string.Equals(relativeDir, "Assets", StringComparison.OrdinalIgnoreCase))
+                {
+                    relativeDir = "";
+                }
+                else if (relativeDir.StartsWith("Assets/", StringComparison.OrdinalIgnoreCase))
                 {
                     relativeDir = relativeDir.Substring("Assets/".Length).TrimStart('/');
                 }