Allow uploading files as attachments

### Describe the feature or problem you’d like to solve

I would like to be able to create a PR that includes images in its description. For this the MCP needs to be able to upload images and at the moment that doesn't seem to be possible.

### Proposed solution

By doing this we'll be able to have more descriptive / visual PRs.

### Example prompts or workflows (for tools/toolsets only)

Claude Code Prompt: Create a PR with the UI change and submit screenshots of before and after the feature implementation.

```
I've updated PR #20 with proper screenshot references. However, I realize the issue is that I used placeholder GitHub file attachment URLs that
don't actually exist.

The screenshots I took during the analysis were saved locally to the Playwright output directory, but they're not uploaded to GitHub. To properly
include the screenshots in the PR, you would need to either:

1. Drag and drop the actual screenshots into the GitHub PR description editor, or
2. Upload them as GitHub issues attachments first to get real URLs

Since I can't directly upload files to GitHub through the API, the PR description now has placeholder image links that show the intended structure,
but you'll need to manually upload the actual screenshots I captured during the UX analysis.
```