Add --tree-only option to output folder structure without file contents

[Copilot]

This PR adds a new --tree-only command-line option that allows users to get only the directory structure tree without processing file contents. This addresses the need for a faster way to explore directory structures when file contents are not needed.

Changes

• New CLI option: Added --tree-only flag that outputs only the folder structure
• Performance optimization: Skips file content collection entirely when tree-only mode is enabled
• Clean output: Tree-only mode outputs the raw tree structure without markdown formatting
• Backward compatibility: All existing functionality remains unchanged

Usage

# Get only the directory tree structure
gpt-copy /path/to/directory --tree-only

# Works with all existing options
gpt-copy /path/to/directory --tree-only --exclude "tests/*"

# Output to file
gpt-copy /path/to/directory --tree-only -o structure.txt

Example Output

Normal mode:

# Folder Structure

project/
├── src/
│ └── main.py
└── docs/
└── README.md

## File: `src/main.py`
*(Relative Path: `src/main.py`)*

```python
1: print("Hello World")

**Tree-only mode:**

project/
├── src/
│ └── main.py
└── docs/
└── README.md

## Benefits

- **Faster execution** for large directories when only structure visualization is needed
- **Cleaner output** without markdown headers and file contents
- **Resource efficient** - doesn't read file contents or perform binary detection
- **Compatible** with existing filtering options (--include, --exclude, --force)

Fixes #6.

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