AX Agent 프리뷰 패널 렌더 구조 분리 및 문서 갱신

- ChatWindow.PreviewPresentation.cs를 추가해 프리뷰 탭 목록, 헤더, 파일 로드, CSV·텍스트·마크다운·HTML 표시, 컨텍스트 메뉴, 별도 창 미리보기 흐름을 메인 창 코드에서 분리함

- ChatWindow.xaml.cs는 transcript 및 런타임 orchestration 중심으로 더 정리해 claw-code 기준 preview surface 품질 개선을 이어가기 쉬운 구조로 개선함

- README.md와 docs/DEVELOPMENT.md에 2026-04-06 08:47 (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

@@ -4899,3 +4899,5 @@ ow + toggle ?쒓컖 ?몄뼱濡??ㅼ떆 ?뺣젹?덈떎.
 - Document update: 2026-04-06 08:27 (KST) - This pass further reduces renderer responsibility in the main chat window file and moves AX Agent closer to the `claw-code` structure where transcript orchestration and message interaction presentation are separated.
 - Document update: 2026-04-06 08:39 (KST) - Moved runtime status bar event handling and spinner animation out of `ChatWindow.xaml.cs` into `ChatWindow.StatusPresentation.cs`. `UpdateStatusBar`, `StartStatusAnimation`, and `StopStatusAnimation` now live alongside the existing operational status presentation helpers.
 - 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.