fix: reject negative --index in ExcelHandler.Add and PowerPointHandler.Add

src/officecli/Handlers/Excel/ExcelHandler.Add.cs

@@ -20,6 +20,14 @@ public string Add(string parentPath, string type, InsertPosition? position, Dict
     {
         Modified = true;
         var index = position?.Index;
+
+        // Reject negative --index up front with a clean message instead of
+        // letting it fall through and surface as a raw .NET
+        // ArgumentOutOfRangeException from collection indexing. Matches
+        // the guard in WordHandler.Add.
+        if (index.HasValue && index.Value < 0)
+            throw new ArgumentException("--index must be non-negative.");
+
         // Normalize to case-insensitive lookup so camelCase keys (e.g. minColor) match lowercase lookups.
         // Preserve TrackingPropertyDictionary so handler-as-truth read
         // tracking survives — its comparer wraps OrdinalIgnoreCase already.

src/officecli/Handlers/Pptx/PowerPointHandler.Add.cs

@@ -38,6 +38,13 @@ public string Add(string parentPath, string type, InsertPosition? position, Dict
         parentPath = ResolveIdPath(parentPath);
         parentPath = ResolveLastPredicates(parentPath);
 
+        // Reject negative --index up front with a clean message instead of
+        // letting it fall through and surface as a raw .NET
+        // ArgumentOutOfRangeException from collection indexing. Matches
+        // the guard in WordHandler.Add.
+        if (position?.Index.HasValue == true && position.Index.Value < 0)
+            throw new ArgumentException("--index must be non-negative.");
+
         // Resolve --after/--before to index (handles find: prefix)
         var index = ResolveAnchorPosition(parentPath, position);