limactl diff command

When managing Lima instances declaratively (using the template file or user-provided config file) or auditing VMs created via --set, users might need to verify how the running state differs from a reference configuration or template.

Currently, there is no easy way to perform this comparison. A simple diff against the running config (inspected via limactl list --json) is insufficient because the running configuration is fully expanded (including defaults and base templates), while local templates are concise.

This limactl diff command would allow users to compare the configuration of a running Lima instance against a local YAML configuration file or template.

Proposed Syntax:

```
limactl diff
```