fix: optimize diagnosis output format and fix inconsistency between fault_status and fault_type

1. 输出格式优化:
   - faults中移除冗余的bbox和confidence字段
   - summary中fault_count改名为detection_count

2. fault_status与fault_type逻辑统一:
   - normal状态时fault_type为'none'，不再使用'suspect'
   - abnormal状态时才进行故障类型分类(hotspot/local_overheating/overheat)

3. 修复usage_example.py图像通道问题:
   - YOLO检测使用彩色图，温度提取时再转灰度图

README.md

@@ -279,8 +279,6 @@ print(f"故障类型: {result['summary']['fault_types']}")
   ],
   "faults": [
     {
-      "bbox": [x1, y1, x2, y2],
-      "confidence": 0.995,
       "temp_max": 255.0,
       "temp_mean": 97.49,
       "temp_std": 45.2,
@@ -290,7 +288,7 @@ print(f"故障类型: {result['summary']['fault_types']}")
   ],
   "summary": {
     "status": "fault_detected",
-    "fault_count": 1,
+    "detection_count": 1,
     "abnormal_count": 1,
     "fault_types": ["hotspot"]
   }