feature: add on-behalf-of: keyword auto-completion

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/Views/CommitMessageToolBox.axaml.cs

@@ -334,7 +334,7 @@ private void OnTextViewVisualLinesChanged(object sender, EventArgs e)
             InvalidateVisual();
         }
 
-        private readonly List<string> _keywords = ["Acked-by: ", "Co-authored-by: ", "Reviewed-by: ", "Signed-off-by: ", "BREAKING CHANGE: ", "Refs: "];
+        private readonly List<string> _keywords = ["Acked-by: ", "Co-authored-by: ", "Reviewed-by: ", "Signed-off-by: ", "on-behalf-of: @", "BREAKING CHANGE: ", "Refs: "];
         private bool _isEditing = false;
         private int _subjectEndLine = 0;
         private CompletionWindow _completionWnd = null;