에이전트 루프 진단과 문서 golden 회귀를 마감한다

- AgentLoopService의 컨텍스트 압축 완료 메시지와 query-view 요약 문자열 조립을 AgentLoopDiagnosticsFormatter로 분리해 루프 본체의 책임을 더 줄였다.

- ChatStorageService 로드 경로에서 legacy .axchat의 누락된 tool_result preview를 synthetic preview로 즉시 복원하도록 보강했다.

- HTML/DOCX golden 회귀와 legacy 저장 경로 회귀 테스트를 추가해 PPTX/XLSX에 이어 문서 품질 고정 범위를 확대했다.

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

- 검증: dotnet test src/AxCopilot.Tests/AxCopilot.Tests.csproj -c Release -v minimal --filter AgentLoopDiagnosticsFormatterTests|ChatStorageServiceTests|HtmlSkillGoldenReportTests|DocxSkillGoldenDocumentTests|AgentMessageInvariantHelperTests|PptxSkillGoldenDeckTests|ExcelSkillGoldenWorkbookTests -p:OutputPath=bin\verify_loop_storage_golden_tests\ -p:IntermediateOutputPath=obj\verify_loop_storage_golden_tests\ (통과 10)

src/AxCopilot.Tests/Services/AgentLoopDiagnosticsFormatterTests.cs

@@ -0,0 +1,47 @@
+using AxCopilot.Services.Agent;
+using FluentAssertions;
+using Xunit;
+
+namespace AxCopilot.Tests.Services;
+
+public class AgentLoopDiagnosticsFormatterTests
+{
+    [Fact]
+    public void BuildCompactionCompleteMessage_ShouldIncludeStageSummaryAndSavedTokens()
+    {
+        var result = new ContextCompactionResult
+        {
+            BeforeTokens = 18_400,
+            AfterTokens = 7_900,
+        };
+        result.AppliedStages.Add("tool-result");
+        result.AppliedStages.Add("session-memory");
+
+        var message = AgentLoopDiagnosticsFormatter.BuildCompactionCompleteMessage(result);
+
+        message.Should().Be("컨텍스트 압축 완료 — tool-result -> session-memory · 10.3K tokens 절감");
+    }
+
+    [Fact]
+    public void BuildQueryViewSummary_ShouldIncludeWindowAndPreviewReuseMetrics()
+    {
+        var result = new AgentQueryContextWindowResult
+        {
+            Messages = new(),
+            SourceMessageCount = 34,
+            ViewMessageCount = 12,
+            WindowStartIndex = 21,
+            BoundaryApplied = true,
+            ToolPairExpanded = true,
+            PreservedToolPairCount = 2,
+            TruncatedToolResultCount = 3,
+            ReusedToolResultPreviewCount = 4,
+            TokensBeforeBudget = 8_600,
+            TokensAfterBudget = 6_100,
+        };
+
+        var summary = AgentLoopDiagnosticsFormatter.BuildQueryViewSummary(result);
+
+        summary.Should().Be("query-view 34->12, start=21, pairs=2, tool_result_budget=3, tool_result_preview_reuse=4, tokens 8600->6100");
+    }
+}