fix(mcp): stop set_list_config from clearing settings it doesn't expose

src/ClaudeDo.Worker/External/ConfigMcpTools.cs

@@ -75,7 +75,9 @@ public sealed class ConfigMcpTools
     [McpServerTool, Description(
         "Set a list's default model/system prompt/agent path/max turns — the fallback for tasks in this list " +
         "that don't override them. Only the fields you pass are changed; omitted fields keep their current " +
-        "value. To clear a field instead, name it in clearFields; clearing all four deletes the list's config.")]
+        "value. To clear a field instead, name it in clearFields; clearing all four deletes the list's config " +
+        "unless it also carries settings this tool doesn't expose (session skills, verify command, file-scope " +
+        "serialization), which are always preserved.")]
     public async Task<SetListConfigResult> SetListConfig(
         string listId, string? model = null, string? systemPrompt = null, string? agentPath = null,
         int? maxTurns = null,
@@ -95,13 +97,23 @@ public sealed class ConfigMcpTools
         var ap = clear.Contains("agentPath") ? null : agentPath.NullIfBlank() ?? existing?.AgentPath;
         var mt = clear.Contains("maxTurns") ? null : maxTurns ?? existing?.MaxTurns;
 
+        // Fields this tool doesn't expose but that live on the same row. They must survive every
+        // write here — ListRepository.SetConfigAsync copies the entity verbatim, so anything left
+        // at its default would silently reset (SerializeOnFileOverlap in particular has no UI
+        // affordance at all, so a reset is invisible until tasks stop serializing).
+        var hasUnrelatedSettings = existing is not null
+            && (existing.SessionSkills is not null
+                || existing.VerifyCommand is not null
+                || existing.SerializeOnFileOverlap);
+
         TaskConfigDto? config;
-        if (m is null && sp is null && ap is null && mt is null && clear.Count > 0)
+        var allCleared = m is null && sp is null && ap is null && mt is null;
+        if (allCleared && clear.Count > 0 && !hasUnrelatedSettings)
         {
             await _lists.DeleteConfigAsync(listId, cancellationToken);
             config = null;
         }
-        else if (m is null && sp is null && ap is null && mt is null && existing is null)
+        else if (allCleared && existing is null)
         {
             config = null;
         }
@@ -111,8 +123,9 @@ public sealed class ConfigMcpTools
             {
                 ListId = listId, Model = m, SystemPrompt = sp, AgentPath = ap, MaxTurns = mt,
                 SessionSkills = existing?.SessionSkills, VerifyCommand = existing?.VerifyCommand,
+                SerializeOnFileOverlap = existing?.SerializeOnFileOverlap ?? false,
             }, cancellationToken);
-            config = new TaskConfigDto(m, sp, ap, mt);
+            config = allCleared ? null : new TaskConfigDto(m, sp, ap, mt);
         }
 
         await _broadcaster.ListUpdated(listId);