`container image tree` command for showing image parentage.

It would be useful to have a command like:

```bash
container image tree
```

Instead of showing a flat list of images, it could display parent-child relationships.

Example:

```text
ubuntu:24.04
├── python:3.12
│ ├── my-api
│ └── worker
└── node:22
└── frontend
```

When you have many locally built images, `container image ls` becomes hard to navigate. A tree view would make it much easier to understand which images are based on others and help with cleanup and debugging. It could also optionally show image sizes with a `--size` flag.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start by locating the existing implementation and entry point for `container image ls`, then trace how locally built images expose parent relationships. Define the tree output and the optional `--size` behavior, including how images without a known parent are displayed. Done means `container image tree` presents image parentage and supports the proposed size display.

Written by the indexing model from the issue text.