feat: 新增 PPT 旧格式支持，重构 LibreOffice 转换工具

- 新增 PPT (旧格式) 解析器
- 重构 _utils.py，提取通用 convert_via_libreoffice 函数
- 更新依赖配置，添加 PPT 相关依赖
- 完善文档，更新 README 和 SKILL.md
- 添加 PPT 文件检测函数
- 新增 PPT 解析器测试用例

scripts/config.py

@@ -116,5 +116,30 @@ DEPENDENCIES = {
             "python": None,
             "dependencies": []
         }
+    },
+    "ppt": {
+        "default": {
+            "python": None,
+            "dependencies": [
+                "docling",
+                "unstructured[pptx]",
+                "markitdown[pptx]",
+                "python-pptx",
+                "markdownify",
+                "olefile"
+            ]
+        },
+        "Darwin-x86_64": {
+            "python": "3.12",
+            "dependencies": [
+                "docling==2.40.0",
+                "docling-parse==4.0.0",
+                "numpy<2",
+                "markitdown[pptx]",
+                "python-pptx",
+                "markdownify",
+                "olefile"
+            ]
+        }
     }
 }