AX Agent 설정/오버레이의 PlanMode dead UI를 제거

- SettingsWindow와 ChatWindow에서 숨김 PlanMode row, hidden toggle, dead handler와 binding 제거

- SettingsViewModel dead PlanMode property/save 경로 정리로 현재 정책과 설정 모델을 일치시킴

- README와 DEVELOPMENT 문서에 2026-04-05 21:12 (KST) 기준 변경 내역과 parity 98% 재평가 반영

- 검증: dotnet build src/AxCopilot/AxCopilot.csproj -c Release -v minimal -p:OutputPath=bin\\verify\\ -p:IntermediateOutputPath=obj\\verify\\ (경고 0 / 오류 0)

src/AxCopilot/ViewModels/SettingsViewModel.cs

@@ -370,15 +370,6 @@ public class SettingsViewModel : INotifyPropertyChanged
         set { _agentDecisionLevel = value; OnPropertyChanged(); }
     }
 
-    public string PlanMode
-    {
-        get => "off";
-        set
-        {
-            OnPropertyChanged();
-        }
-    }
-
     private bool _enableMultiPassDocument;
     public bool EnableMultiPassDocument
     {
@@ -1581,7 +1572,6 @@ public class SettingsViewModel : INotifyPropertyChanged
         s.Llm.PlanDiffSeverityMediumRatioPercent = _planDiffSeverityMediumRatioPercent;
         s.Llm.PlanDiffSeverityHighRatioPercent = _planDiffSeverityHighRatioPercent;
         s.Llm.AgentDecisionLevel     = _agentDecisionLevel;
-        s.Llm.PlanMode               = "off";
         s.Llm.EnableMultiPassDocument = _enableMultiPassDocument;
         s.Llm.EnableCoworkVerification = _enableCoworkVerification;
         s.Llm.EnableFilePathHighlight = _enableFilePathHighlight;