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>

operation/repo/file.go

@@ -16,6 +16,7 @@ import (
 
 const (
 	GetFileToolName    = "get_file_content"
+	GetDirToolName     = "get_dir_content"
 	CreateFileToolName = "create_file"
 	UpdateFileToolName = "update_file"
 	DeleteFileToolName = "delete_file"
@@ -31,6 +32,15 @@ var (
 		mcp.WithString("filePath", mcp.Required(), mcp.Description("file path")),
 	)
 
+	GetDirContentTool = mcp.NewTool(
+		GetDirToolName,
+		mcp.WithDescription("Get a list of entries in a directory"),
+		mcp.WithString("owner", mcp.Required(), mcp.Description("repository owner")),
+		mcp.WithString("repo", mcp.Required(), mcp.Description("repository name")),
+		mcp.WithString("ref", mcp.Required(), mcp.Description("ref can be branch/tag/commit")),
+		mcp.WithString("filePath", mcp.Required(), mcp.Description("directory path")),
+	)
+
 	CreateFileTool = mcp.NewTool(
 		CreateFileToolName,
 		mcp.WithDescription("Create file"),
@@ -72,6 +82,10 @@ func init() {
 		Tool:    GetFileContentTool,
 		Handler: GetFileContentFn,
 	})
+	Tool.RegisterRead(server.ServerTool{
+		Tool:    GetDirContentTool,
+		Handler: GetDirContentFn,
+	})
 	Tool.RegisterWrite(server.ServerTool{
 		Tool:    CreateFileTool,
 		Handler: CreateFileFn,
@@ -108,6 +122,28 @@ func GetFileContentFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallTo
 	return to.TextResult(content)
 }
 
+func GetDirContentFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+	log.Debugf("Called GetDirContentFn")
+	owner, ok := req.GetArguments()["owner"].(string)
+	if !ok {
+		return to.ErrorResult(fmt.Errorf("owner is required"))
+	}
+	repo, ok := req.GetArguments()["repo"].(string)
+	if !ok {
+		return to.ErrorResult(fmt.Errorf("repo is required"))
+	}
+	ref, _ := req.GetArguments()["ref"].(string)
+	filePath, ok := req.GetArguments()["filePath"].(string)
+	if !ok {
+		return to.ErrorResult(fmt.Errorf("filePath is required"))
+	}
+	content, _, err := gitea.Client().ListContents(owner, repo, ref, filePath)
+	if err != nil {
+		return to.ErrorResult(fmt.Errorf("get dir content err: %v", err))
+	}
+	return to.TextResult(content)
+}
+
 func CreateFileFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 	log.Debugf("Called CreateFileFn")
 	owner, ok := req.GetArguments()["owner"].(string)