tool_result preview 복원과 슬래시 명령 합성 일원화

목적:
- 긴 세션, 분기, 재시작 이후에도 tool_result preview 축약 상태를 더 안정적으로 유지합니다.
- 슬래시 팔레트와 실제 /토큰 실행 해석이 어긋나지 않도록 built-in command와 skill 우선순위를 같은 규칙으로 맞춥니다.

핵심 수정:
- AgentMessageInvariantHelper에 tool_use_id 기준 preview 맵/복원 helper를 추가했습니다.
- ChatSessionStateService는 분기 대화 생성 시 QueryPreviewContent를 함께 복사하고, 저장된 대화를 다시 열 때 누락된 preview를 복원합니다.
- ChatStorageService는 저장 직전에 누락된 tool_result preview를 먼저 채워 재시작 후 축약 상태가 흔들리지 않게 정리했습니다.
- SlashCommandCatalog에 exact token 충돌 해석용 ResolvePreferredCommand를 추가하고, ChatWindow.ParseSlashCommandAsync가 built-in/skill 후보를 함께 모아 같은 우선순위 규칙으로 실행 대상을 선택하도록 맞췄습니다.
- SlashCommandCatalogTests를 새로 추가하고 ChatSessionStateServiceTests를 확장해 preview 복원과 skill 우선 해석을 회귀 검증했습니다.
- README.md, docs/DEVELOPMENT.md를 2026-04-15 07:16 (KST) 기준으로 갱신했습니다.

검증:
- dotnet build src/AxCopilot/AxCopilot.csproj -c Release -v minimal -p:OutputPath=bin\\verify_preview_state\\ -p:IntermediateOutputPath=obj\\verify_preview_state\\
- dotnet test src/AxCopilot.Tests/AxCopilot.Tests.csproj -c Release -v minimal --filter "AgentToolResultBudgetTests|ChatSessionStateServiceTests" -p:OutputPath=bin\\verify_preview_state_tests\\ -p:IntermediateOutputPath=obj\\verify_preview_state_tests\\ (통과 38)
- dotnet build src/AxCopilot/AxCopilot.csproj -c Release -v minimal -p:OutputPath=bin\\verify_command_resolution\\ -p:IntermediateOutputPath=obj\\verify_command_resolution\\
- dotnet test src/AxCopilot.Tests/AxCopilot.Tests.csproj -c Release -v minimal --filter "SlashCommandCatalogTests|ChatSessionStateServiceTests|AgentToolResultBudgetTests|AgentCommandQueueTests" -p:OutputPath=bin\\verify_command_resolution_tests\\ -p:IntermediateOutputPath=obj\\verify_command_resolution_tests\\ (통과 50)

src/AxCopilot/Services/Agent/AgentMessageInvariantHelper.cs

@@ -8,6 +8,51 @@ namespace AxCopilot.Services.Agent;
 /// </summary>
 internal static class AgentMessageInvariantHelper
 {
+    public static Dictionary<string, string> BuildToolResultPreviewMap(IEnumerable<ChatMessage>? messages)
+    {
+        var previews = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        if (messages == null)
+            return previews;
+
+        foreach (var message in messages)
+        {
+            if (string.IsNullOrWhiteSpace(message.QueryPreviewContent))
+                continue;
+            if (!TryGetToolResultId(message, out var toolResultId))
+                continue;
+
+            previews[toolResultId] = message.QueryPreviewContent!;
+        }
+
+        return previews;
+    }
+
+    public static bool PopulateMissingToolResultPreviews(IList<ChatMessage>? messages)
+    {
+        if (messages == null || messages.Count == 0)
+            return false;
+
+        var previews = BuildToolResultPreviewMap(messages);
+        if (previews.Count == 0)
+            return false;
+
+        var changed = false;
+        foreach (var message in messages)
+        {
+            if (!string.IsNullOrWhiteSpace(message.QueryPreviewContent))
+                continue;
+            if (!TryGetToolResultId(message, out var toolResultId))
+                continue;
+            if (!previews.TryGetValue(toolResultId, out var preview))
+                continue;
+
+            message.QueryPreviewContent = preview;
+            changed = true;
+        }
+
+        return changed;
+    }
+
     public static int AdjustStartIndexForToolPairs(
         IReadOnlyList<ChatMessage> messages,
         int startIndex,

src/AxCopilot/Services/Agent/AgentToolResultBudget.cs

@@ -30,7 +30,7 @@ public static class AgentToolResultBudget
         var sourceById = sourceMessages?
             .Where(message => !string.IsNullOrWhiteSpace(message.MsgId))
             .ToDictionary(message => message.MsgId, StringComparer.OrdinalIgnoreCase);
-        var sourcePreviewByToolResultId = BuildPreviewByToolResultId(sourceMessages);
+        var sourcePreviewByToolResultId = AgentMessageInvariantHelper.BuildToolResultPreviewMap(sourceMessages);
         var nonSystemIndexes = messages
             .Select((message, index) => new { message, index })
             .Where(x => !string.Equals(x.message.Role, "system", StringComparison.OrdinalIgnoreCase))
@@ -100,25 +100,6 @@ public static class AgentToolResultBudget
         return result;
     }
 
-    private static Dictionary<string, string> BuildPreviewByToolResultId(IReadOnlyList<ChatMessage>? sourceMessages)
-    {
-        var previews = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
-        if (sourceMessages == null)
-            return previews;
-
-        foreach (var message in sourceMessages)
-        {
-            if (string.IsNullOrWhiteSpace(message.QueryPreviewContent))
-                continue;
-            if (!AgentMessageInvariantHelper.TryGetToolResultId(message, out var toolResultId))
-                continue;
-
-            previews[toolResultId] = message.QueryPreviewContent!;
-        }
-
-        return previews;
-    }
-
     public static string TruncateToolResultJson(string json, int softCharLimit = DefaultSoftCharLimit)
     {
         try