오래된 tool_result를 시간 기반으로 정리하고 compact 직후 안내를 단순화

- 마지막 assistant 이후 긴 휴지기가 있으면 오래된 tool_result를 cleared marker로 교체하는 time-based microcompact 성격의 분기 추가
- compact 직후 첫 턴에서 운영성 thinking 문구를 다시 띄우지 않도록 AgentLoopCompactionPolicy 정리
- README와 DEVELOPMENT 문서에 2026-04-12 22:11 (KST) 기준 작업 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)

src/AxCopilot/Services/Agent/ContextCondenser.cs

@@ -42,6 +42,9 @@ public static class ContextCondenser
     private const int RecentKeepCount = 6;
     private const int AutoCompactBufferTokens = 13_000;
     private const int SummaryReserveTokens = 20_000;
+    private const int TimeBasedToolResultGapMinutes = 20;
+    private const int TimeBasedKeepRecentToolResults = 1;
+    private const string TimeBasedClearedToolResultMessage = "[time-based microcompact] 이전 tool_result 내용이 정리되었습니다.";
 
     private sealed class CompactionWindow
     {
@@ -119,8 +122,16 @@ public static class ContextCondenser
 
         var currentTokens = TokenEstimator.EstimateMessages(messages);
         result.BeforeTokens = currentTokens;
+
+        if (ApplyTimeBasedToolResultCompaction(messages, out var clearedToolResults))
+        {
+            result.AppliedStages.Add($"time-gap-tool-result({clearedToolResults})");
+            currentTokens = TokenEstimator.EstimateMessages(messages);
+        }
+
         if (!force && currentTokens < threshold)
         {
+            result.Changed = result.AppliedStages.Count > 0;
             result.AfterTokens = currentTokens;
             return result;
         }
@@ -240,6 +251,55 @@ public static class ContextCondenser
         };
     }
 
+    private static bool ApplyTimeBasedToolResultCompaction(List<ChatMessage> messages, out int clearedCount)
+    {
+        clearedCount = 0;
+        var lastAssistant = messages
+            .Where(m => string.Equals(m.Role, "assistant", StringComparison.OrdinalIgnoreCase))
+            .OrderByDescending(m => m.Timestamp)
+            .FirstOrDefault();
+
+        if (lastAssistant?.Timestamp is not DateTime lastAssistantAt)
+            return false;
+
+        var gapMinutes = (DateTime.Now - lastAssistantAt).TotalMinutes;
+        if (!double.IsFinite(gapMinutes) || gapMinutes < TimeBasedToolResultGapMinutes)
+            return false;
+
+        var candidateIndexes = messages
+            .Select((message, index) => new { message, index })
+            .Where(x => AgentMessageInvariantHelper.TryGetToolResultId(x.message, out _))
+            .Select(x => x.index)
+            .ToList();
+
+        if (candidateIndexes.Count <= TimeBasedKeepRecentToolResults)
+            return false;
+
+        var keepSet = candidateIndexes
+            .TakeLast(Math.Max(1, TimeBasedKeepRecentToolResults))
+            .ToHashSet();
+
+        foreach (var index in candidateIndexes)
+        {
+            if (keepSet.Contains(index))
+                continue;
+
+            var message = messages[index];
+            var content = message.Content ?? "";
+            if (string.IsNullOrWhiteSpace(content))
+                continue;
+
+            var cleared = AgentToolResultBudget.CreateClearedToolResultJson(content, TimeBasedClearedToolResultMessage);
+            if (string.Equals(cleared, content, StringComparison.Ordinal))
+                continue;
+
+            messages[index] = CloneWithContent(message, cleared);
+            clearedCount++;
+        }
+
+        return clearedCount > 0;
+    }
+
     /// <summary>
     /// 1단계: 오래된 tool_result는 aggregate budget 기준으로 먼저 줄이고,
     /// 그 외 긴 assistant/user 메시지는 경량 절단합니다.