feat: add GetDirContent tool for retrieving directory entries (#53)

### 🚀 What's Changed
This PR introduces a new MCP tool `get_dir_content` that allows users to retrieve a list of entries (files and subdirectories) from a specified directory in a Gitea repository.

### ✨ Features Added
- **New Tool**: `GetDirContent` tool for directory listing functionality
- **Tool Registration**: Properly registered as a read operation in the MCP server
- **Parameter Validation**: Comprehensive input validation for required parameters
- **Error Handling**: Robust error handling with descriptive error messages

### 🔧 Technical Details
- **Tool Name**: `get_dir_content`
- **Required Parameters**:
  - `owner`: Repository owner
  - `repo`: Repository name
  - `ref`: Branch, tag, or commit reference
  - `filePath`: Directory path to list

### 📁 Files Modified
- file.go: Added tool definition, registration, and handler function

### 🎯 Use Cases
This tool enables users to:
- Browse repository directory structures
- List files and folders in specific directories
- Navigate repository contents programmatically
- Support file management workflows in MCP clients

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/53
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: ZRE <chy853@gmail.com>
Co-committed-by: ZRE <chy853@gmail.com>

README.zh-cn.md

@@ -194,6 +194,7 @@ Gitea MCP 服务器支持以下工具：
 |          list_tags           |   标签   |         列出所有标签         |
 |      list_repo_commits       |   提交   |     列出仓库中的所有提交     |
 |       get_file_content       |   文件   |    获取文件的内容和元数据    |
+|        get_dir_content       |   文件   |      获取目录的内容列表      |
 |         create_file          |   文件   |        创建一个新文件        |
 |         update_file          |   文件   |         更新现有文件         |
 |         delete_file          |   文件   |         删除一个文件         |