Support migrating to existing GitHub repository

## Problem
Currently, `gh ado2gh migrate-repo` requires creating a new repository and fails if the target repository already exists with the error:
```
The Org 'xxx' already contains a repository with the name 'yyy'. No operation will be performed
```

## Use Case
We have already created an empty GitHub repository and want to migrate our Azure DevOps repository (including PRs, comments, history) into this existing repository.

## Current Workaround
The only option is to:
1. Delete the existing GitHub repository
2. Run the migration (which creates a new repo)

## Requested Feature
Add support for migrating into an existing GitHub repository, either:
- Option A: Add a `--overwrite` or `--force` flag to replace existing repo contents
- Option B: Add a `--target-existing` flag to migrate into existing empty repos
- Option C: Support migration without requiring repository creation

## Environment
- CLI Version: 1.26.0
- Source: Azure DevOps
- Target: GitHub Enterprise Cloud

Is there any existing way to accomplish this, or would this be possible to add?