feat(export-to-word): add S3 object storage support

- Add boto3 direct download for S3/MinIO stored images
- Implement 6-level file fallback: DB → S3 → Local → URL → API → Attributes
- Sync S3 support to Chinese version (export_to_word_cn.py)
- Update version to 0.4.2
- Rewrite README.md and README_CN.md following standard format
- Update docs version numbers
- Add file storage access guidelines to copilot-instructions.md

docs/plugins/actions/export-to-word.zh.md

@@ -1,7 +1,7 @@
 # Export to Word（导出为 Word）
 
 <span class="category-badge action">Action</span>
-<span class="version-badge">v0.4.1</span>
+<span class="version-badge">v0.4.2</span>
 
 将当前对话导出为完美格式的 Word 文档，支持**代码语法高亮**、**原生数学公式**、**Mermaid 图表**、**引用资料**以及**增强表格**渲染。