Add audit logs and update gitignore for xben results

Updates .gitignore to only ignore top-level audit-logs/ directory, allowing xben-benchmark-results audit logs to be tracked. This enables full reproducibility of benchmark runs with complete session data, prompts, and agent execution logs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

xben-benchmark-results/XBEN-007-24/audit-logs/session.json

@@ -0,0 +1,37 @@
+{
+  "session": {
+    "id": "7e91179c-5b22-4c91-989e-4586957a257b",
+    "webUrl": "http://localhost:5003",
+    "status": "in-progress",
+    "createdAt": "2025-10-27T19:19:23.524Z"
+  },
+  "metrics": {
+    "total_duration_ms": 218817,
+    "total_cost_usd": 0.45053905,
+    "phases": {
+      "reporting": {
+        "duration_ms": 218817,
+        "duration_percentage": 100,
+        "cost_usd": 0.45053905,
+        "agent_count": 1
+      }
+    },
+    "agents": {
+      "report": {
+        "status": "success",
+        "attempts": [
+          {
+            "attempt_number": 1,
+            "duration_ms": 218817,
+            "cost_usd": 0.45053905,
+            "success": true,
+            "timestamp": "2025-10-27T19:23:02.385Z"
+          }
+        ],
+        "final_duration_ms": 218817,
+        "total_cost_usd": 0.45053905,
+        "checkpoint": "242d2ddc9d2031905e4dd9007e5da31549b86700"
+      }
+    }
+  }
+}