post-compaction 추적과 대화별 사용량 계측 보강

- claude-code의 post-compaction 플래그 흐름을 참고해 compact 직후 첫 응답을 별도로 추적하고 대화 단위 후속 사용량을 집계함

- ChatConversation에 pending post-compaction, 응답 회수, prompt/completion token 누적 필드를 추가하고 AX Agent 컨텍스트 카드 hover에 표시함

- README 및 docs/DEVELOPMENT.md에 2026-04-04 23:47 (KST) 기준 이력을 반영함

- 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/Models/ChatModels.cs

@@ -175,6 +175,18 @@ public class ChatConversation
 
     [JsonPropertyName("lastCompactionStageSummary")]
     public string? LastCompactionStageSummary { get; set; }
+
+    [JsonPropertyName("pendingPostCompaction")]
+    public bool PendingPostCompaction { get; set; }
+
+    [JsonPropertyName("postCompactionResponseCount")]
+    public int PostCompactionResponseCount { get; set; }
+
+    [JsonPropertyName("postCompactionPromptTokens")]
+    public int PostCompactionPromptTokens { get; set; }
+
+    [JsonPropertyName("postCompactionCompletionTokens")]
+    public int PostCompactionCompletionTokens { get; set; }
 }
 
 public class ChatAgentRunRecord