Introduce `nemoclaw backup||restore config`

### Problem Statement

Current `nemoclaw snapshot` functionality only takes care of a few key folders. The config is left untouched.
At the same time, we've seen some work being done to allow us to modify part of the config.

Why not let us replace the full config? That way we keep the sandbox neutral and tune it via full config files.

- It's already possible today via custom scripts using OpenShell, but it would be cleaner to be able to use a `nemoclaw` command.
- The same principle is already available for network policies. Bake in a blank one and upload the one needed afterwards.

### Proposed Design

Commands
- `nemoclaw backup config`
- `nemoclaw restore config`

When restoring, the following has to be performed:
- Backup existing config
- Validate schema of new config against the version used in the sandbox
- Migrate config elements if such changes are tracked ( doctor --fix?)

### Alternatives Considered

It's possible today to write scripts to automate these tasks.

### Category

enhancement: feature

### Checklist

- [x] I searched existing issues and this is not a duplicate
- [x] This is a design proposal, not a "please build this" request