AX Agent 좌측 패널 드래그 리사이즈 지원 추가

- 좌측 사이드바와 본문 사이 경계선에 GridSplitter 추가
- 사이드바가 열려 있을 때만 드래그 가능하도록 splitter 표시 상태 연동
- 사용자가 조절한 폭을 저장해 사이드바를 닫았다 다시 열어도 마지막 너비 유지
- 열기/닫기 애니메이션이 현재 폭과 저장 폭을 함께 사용하도록 정리
- README 및 DEVELOPMENT 문서에 2026-04-05 23:15 (KST) 기준 변경 이력 반영

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

src/AxCopilot/Views/ChatWindow.xaml

@@ -837,6 +837,20 @@
                 </Grid>
             </Border>
 
+            <GridSplitter x:Name="SidebarResizeSplitter"
+                          Grid.Column="1"
+                          Width="8"
+                          HorizontalAlignment="Right"
+                          VerticalAlignment="Stretch"
+                          Background="Transparent"
+                          ResizeDirection="Columns"
+                          ResizeBehavior="PreviousAndNext"
+                          ShowsPreview="False"
+                          Cursor="SizeWE"
+                          Visibility="Visible"
+                          DragCompleted="SidebarResizeSplitter_DragCompleted"
+                          Panel.ZIndex="40"/>
+
             <!-- ══════════════════════════════════════════════════════ -->
             <!-- 우측: 메시지 영역                                       -->
             <!-- ══════════════════════════════════════════════════════ -->

src/AxCopilot/Views/ChatWindow.xaml.cs

@@ -34,6 +34,7 @@ public partial class ChatWindow : Window
     private CancellationTokenSource? _streamCts;
     private bool _isStreaming;
     private bool _sidebarVisible = true;
+    private double _sidebarExpandedWidth = 262;
     private string _selectedCategory = ""; // "" = 전체
     private readonly Dictionary<string, string> _tabSelectedCategory = new(StringComparer.OrdinalIgnoreCase)
     {
@@ -2854,25 +2855,33 @@ public partial class ChatWindow : Window
             IconBarColumn.Width = new GridLength(0);
             IconBarPanel.Visibility = Visibility.Collapsed;
             SidebarPanel.Visibility = Visibility.Visible;
+            SidebarResizeSplitter.Visibility = Visibility.Visible;
+
+            var targetWidth = Math.Max(190, _sidebarExpandedWidth);
 
             if (animated)
             {
-            AnimateSidebar(0, 220, () => SidebarColumn.MinWidth = 168);
+                AnimateSidebar(0, targetWidth, () => SidebarColumn.MinWidth = 168);
             }
             else
             {
-            SidebarColumn.MinWidth = 168;
-            SidebarColumn.Width = new GridLength(220);
+                SidebarColumn.MinWidth = 168;
+                SidebarColumn.Width = new GridLength(targetWidth);
             }
             return;
         }
 
+        var currentWidth = SidebarColumn.ActualWidth > 0 ? SidebarColumn.ActualWidth : SidebarColumn.Width.Value;
+        if (currentWidth > 0)
+            _sidebarExpandedWidth = Math.Max(190, currentWidth);
+
         SidebarColumn.MinWidth = 0;
         if (animated)
         {
-            AnimateSidebar(248, 0, () =>
+            AnimateSidebar(currentWidth > 0 ? currentWidth : _sidebarExpandedWidth, 0, () =>
             {
                 SidebarPanel.Visibility = Visibility.Collapsed;
+                SidebarResizeSplitter.Visibility = Visibility.Collapsed;
                 IconBarColumn.Width = new GridLength(52);
                 IconBarPanel.Visibility = Visibility.Visible;
             });
@@ -2881,11 +2890,22 @@ public partial class ChatWindow : Window
         {
             SidebarColumn.Width = new GridLength(0);
             SidebarPanel.Visibility = Visibility.Collapsed;
+            SidebarResizeSplitter.Visibility = Visibility.Collapsed;
             IconBarColumn.Width = new GridLength(52);
             IconBarPanel.Visibility = Visibility.Visible;
         }
     }
 
+    private void SidebarResizeSplitter_DragCompleted(object sender, DragCompletedEventArgs e)
+    {
+        if (!_sidebarVisible)
+            return;
+
+        var currentWidth = SidebarColumn.ActualWidth > 0 ? SidebarColumn.ActualWidth : SidebarColumn.Width.Value;
+        _sidebarExpandedWidth = Math.Clamp(currentWidth, 190, 420);
+        SidebarColumn.Width = new GridLength(_sidebarExpandedWidth);
+    }
+
     private void AnimateSidebar(double from, double to, Action? onComplete = null)
     {
         var duration = 200.0;