AX Agent 파일 브라우저 렌더 구조 분리 및 문서 갱신

- ChatWindow.FileBrowserPresentation.cs를 추가해 파일 탐색기 열기/닫기, 폴더 트리 구성, 파일 헤더·아이콘·크기 표시, 우클릭 메뉴, 디바운스 새로고침 흐름을 메인 창 코드에서 분리함

- ChatWindow.xaml.cs는 transcript·runtime orchestration 중심으로 더 정리해 claw-code 기준 sidebar/file surface 품질 개선을 이어가기 쉬운 구조로 개선함

- README.md와 docs/DEVELOPMENT.md에 2026-04-06 08:55 (KST) 기준 변경 이력을 반영함

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

docs/DEVELOPMENT.md

@@ -4901,3 +4901,5 @@ ow + toggle ?쒓컖 ?몄뼱濡??ㅼ떆 ?뺣젹?덈떎.
 - Document update: 2026-04-06 08:39 (KST) - This pass reduces direct status-line branching in the main chat window file and keeps AX Agent closer to the `claw-code` model where runtime/footer presentation is separated from session orchestration.
 - Document update: 2026-04-06 08:47 (KST) - Split preview surface rendering out of `ChatWindow.xaml.cs` into `ChatWindow.PreviewPresentation.cs`. Preview tab tracking, panel open/close, tab bar rebuilding, preview header state, CSV/text/markdown/HTML loaders, context menu actions, and external popup preview are now grouped in a dedicated partial.
 - Document update: 2026-04-06 08:47 (KST) - This keeps `ChatWindow.xaml.cs` focused on transcript/runtime orchestration and aligns AX Agent more closely with the `claw-code` model where preview/session surfaces are treated as separate presentation layers.
+- Document update: 2026-04-06 08:55 (KST) - Split file browser presentation out of `ChatWindow.xaml.cs` into `ChatWindow.FileBrowserPresentation.cs`. File browser open/close handlers, folder tree population, file item header/icon/size formatting, context menu actions, and debounced refresh logic now live in a dedicated partial.
+- Document update: 2026-04-06 08:55 (KST) - This keeps `ChatWindow.xaml.cs` more orchestration-focused and aligns AX Agent more closely with the `claw-code` model where sidebar/file surfaces are separated from transcript and runtime flow.