AX Agent 실행 준비 로직을 엔진으로 추가 이관

- AxAgentExecutionEngine에 PreparedExecution, PrepareExecution, NormalizeAssistantContent를 추가해 실행 준비와 최종 응답 보정 책임을 더 모음

- ChatWindow의 일반 전송과 재생성이 PrepareExecutionForConversation을 통해 같은 준비 경로를 쓰도록 정리함

- Cowork/Code 시스템 프롬프트 선택, 실행 모드 판정, 프롬프트 스택 조합, outbound message 준비의 중복 분기를 줄임

- 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/Services/Agent/AxAgentExecutionEngine.cs

@@ -12,6 +12,10 @@ public sealed class AxAgentExecutionEngine
 {
     public sealed record PreparedTurn(List<ChatMessage> Messages);
     public sealed record ExecutionMode(bool UseAgentLoop, bool UseStreamingTransport, string? TaskSystemPrompt);
+    public sealed record PreparedExecution(
+        ExecutionMode Mode,
+        IReadOnlyList<string> PromptStack,
+        List<ChatMessage> Messages);
 
     public IReadOnlyList<string> BuildPromptStack(
         string? conversationSystem,
@@ -44,6 +48,32 @@ public sealed class AxAgentExecutionEngine
         return new ExecutionMode(false, false, null);
     }
 
+    public PreparedExecution PrepareExecution(
+        ChatConversation conversation,
+        string runTab,
+        bool streamingEnabled,
+        string resolvedService,
+        string? conversationSystem,
+        string? slashSystem,
+        string? coworkSystemPrompt,
+        string? codeSystemPrompt,
+        string? fileContext = null,
+        IReadOnlyList<ImageAttachment>? images = null)
+    {
+        var mode = ResolveExecutionMode(
+            runTab,
+            streamingEnabled,
+            resolvedService,
+            coworkSystemPrompt,
+            codeSystemPrompt);
+        var promptStack = BuildPromptStack(
+            conversationSystem,
+            slashSystem,
+            mode.TaskSystemPrompt);
+        var preparedTurn = PrepareTurn(conversation, promptStack, fileContext, images);
+        return new PreparedExecution(mode, promptStack, preparedTurn.Messages);
+    }
+
     public PreparedTurn PrepareTurn(
         ChatConversation conversation,
         IEnumerable<string?> systemPrompts,
@@ -103,6 +133,33 @@ public sealed class AxAgentExecutionEngine
         return assistant;
     }
 
+    public string NormalizeAssistantContent(
+        ChatConversation conversation,
+        string runTab,
+        string? content)
+    {
+        if (!string.IsNullOrWhiteSpace(content))
+            return content;
+
+        var latestEventSummary = conversation.ExecutionEvents?
+            .Where(evt => !string.IsNullOrWhiteSpace(evt.Summary))
+            .OrderByDescending(evt => evt.Timestamp)
+            .Select(evt => evt.Summary.Trim())
+            .FirstOrDefault();
+
+        if (!string.IsNullOrWhiteSpace(latestEventSummary))
+        {
+            return runTab switch
+            {
+                "Cowork" => $"코워크 작업이 완료되었습니다.\n\n{latestEventSummary}",
+                "Code" => $"코드 작업이 완료되었습니다.\n\n{latestEventSummary}",
+                _ => latestEventSummary,
+            };
+        }
+
+        return "(빈 응답)";
+    }
+
     private static ChatMessage CloneMessage(ChatMessage source)
     {
         return new ChatMessage