Fix JsonDocument disposal in AgentPolicyEvaluator.ParsePolicy

Add missing `using` on JsonDocument.Parse to prevent resource leak.
Found during adversarial self-review.

Author: Chris0Jeky

backend/src/Taskdeck.Application/Services/AgentPolicyEvaluator.cs

@@ -126,7 +126,7 @@ internal static AgentPolicyConfig ParsePolicy(string? policyJson)
 
         try
         {
-            var doc = JsonDocument.Parse(policyJson);
+            using var doc = JsonDocument.Parse(policyJson);
             var root = doc.RootElement;
 
             var allowedTools = new List<string>();