feat: 为metadata、模板和幻灯片添加description字段支持

添加可选的description字段用于文档目的，不影响渲染输出。

主要更改：
- core/presentation.py: 添加metadata.description属性
- core/template.py: 添加template.description属性
- tests: 添加16个新测试用例验证description功能
- docs: 更新README.md和README_DEV.md文档
- specs: 新增page-description规范文件

tests/unit/test_presentation.py

@@ -600,3 +600,161 @@ slides:
         assert result["elements"][1].content == "Template2"
         assert result["elements"][2].content == "Custom1"
         assert result["elements"][3].content == "Custom2"
+
+
+# ============= Description 字段测试 =============
+
+
+class TestPresentationDescription:
+    """Presentation description 字段测试类"""
+
+    def test_metadata_with_description(self, temp_dir):
+        """测试 metadata 包含 description 字段时正确加载"""
+        yaml_content = """
+metadata:
+  title: "测试演示文稿"
+  description: "这是关于项目年度总结的演示文稿"
+  size: "16:9"
+slides:
+  - elements: []
+"""
+        yaml_path = temp_dir / "test.yaml"
+        yaml_path.write_text(yaml_content)
+
+        pres = Presentation(str(yaml_path))
+
+        assert pres.description == "这是关于项目年度总结的演示文稿"
+
+    def test_metadata_without_description(self, temp_dir):
+        """测试 metadata 不包含 description 字段时正常工作"""
+        yaml_content = """
+metadata:
+  title: "测试演示文稿"
+  size: "16:9"
+slides:
+  - elements: []
+"""
+        yaml_path = temp_dir / "test.yaml"
+        yaml_path.write_text(yaml_content)
+
+        pres = Presentation(str(yaml_path))
+
+        assert pres.description is None
+
+    def test_metadata_description_empty_string(self, temp_dir):
+        """测试 metadata description 为空字符串时正常工作"""
+        yaml_content = """
+metadata:
+  title: "测试演示文稿"
+  description: ""
+  size: "16:9"
+slides:
+  - elements: []
+"""
+        yaml_path = temp_dir / "test.yaml"
+        yaml_path.write_text(yaml_content)
+
+        pres = Presentation(str(yaml_path))
+
+        assert pres.description == ""
+
+    def test_metadata_description_chinese_characters(self, temp_dir):
+        """测试 metadata description 包含中文字符时正确处理"""
+        yaml_content = """
+metadata:
+  title: "测试"
+  description: "这是关于项目的演示文稿，包含中文和特殊字符：测试、验证、确认"
+  size: "16:9"
+slides:
+  - elements: []
+"""
+        yaml_path = temp_dir / "test.yaml"
+        yaml_path.write_text(yaml_content)
+
+        pres = Presentation(str(yaml_path))
+
+        assert "这是关于项目的演示文稿" in pres.description
+        assert "中文" in pres.description
+
+
+class TestSlideDescription:
+    """Slide description 字段测试类"""
+
+    def test_slide_with_description(self, sample_yaml):
+        """测试幻灯片包含 description 字段时正确加载"""
+        pres = Presentation(str(sample_yaml))
+
+        slide_data = {
+            "description": "介绍项目背景和目标",
+            "elements": [
+                {"type": "text", "content": "Test", "box": [0, 0, 1, 1], "font": {}}
+            ]
+        }
+
+        result = pres.render_slide(slide_data)
+
+        assert result["description"] == "介绍项目背景和目标"
+
+    def test_slide_without_description(self, sample_yaml):
+        """测试幻灯片不包含 description 字段时正常工作"""
+        pres = Presentation(str(sample_yaml))
+
+        slide_data = {
+            "elements": [
+                {"type": "text", "content": "Test", "box": [0, 0, 1, 1], "font": {}}
+            ]
+        }
+
+        result = pres.render_slide(slide_data)
+
+        assert result["description"] is None
+
+    def test_slide_description_empty_string(self, sample_yaml):
+        """测试幻灯片 description 为空字符串时正常工作"""
+        pres = Presentation(str(sample_yaml))
+
+        slide_data = {
+            "description": "",
+            "elements": [
+                {"type": "text", "content": "Test", "box": [0, 0, 1, 1], "font": {}}
+            ]
+        }
+
+        result = pres.render_slide(slide_data)
+
+        assert result["description"] == ""
+
+    def test_slide_description_chinese_characters(self, sample_yaml):
+        """测试幻灯片 description 包含中文字符时正确处理"""
+        pres = Presentation(str(sample_yaml))
+
+        slide_data = {
+            "description": "这是幻灯片描述，包含中文内容",
+            "elements": [
+                {"type": "text", "content": "Test", "box": [0, 0, 1, 1], "font": {}}
+            ]
+        }
+
+        result = pres.render_slide(slide_data)
+
+        assert "这是幻灯片描述" in result["description"]
+        assert "中文" in result["description"]
+
+    def test_slide_description_does_not_affect_rendering(self, sample_yaml):
+        """测试 description 不影响渲染输出"""
+        pres = Presentation(str(sample_yaml))
+
+        slide_data = {
+            "description": "这段描述不应该影响渲染",
+            "elements": [
+                {"type": "text", "content": "Test Content", "box": [0, 0, 1, 1], "font": {}}
+            ]
+        }
+
+        result = pres.render_slide(slide_data)
+
+        # description 字段存在但不影响元素渲染
+        assert result["description"] == "这段描述不应该影响渲染"
+        assert len(result["elements"]) == 1
+        assert result["elements"][0].content == "Test Content"
+