Add --latest to start command

### Description

Currently there is an ad-hoc list of the images, with one "release" and one "latest":

```yaml
images:
# Try to use release-yyyyMMdd image if available. Note that release-yyyyMMdd will be removed after several months.

- location: "https://cloud-images.ubuntu.com/releases/questing/release-20260414/ubuntu-25.10-server-cloudimg-***.img"
arch: "***"
digest: "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"

# Fallback to the latest release image.
# Hint: run `limactl prune` to invalidate the cache

- location: https://cloud-images.ubuntu.com/releases/questing/release/ubuntu-25.10-server-cloudimg-***.img
arch: ***
```

```yaml
images:
- location: "https://repo.almalinux.org/almalinux/10.1/cloud/***/images/AlmaLinux-10-GenericCloud-10.1-20260414.0.***.qcow2"
arch: "***"
digest: "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"

# Fallback to the latest release image.
# Hint: run `limactl prune` to invalidate the cache

- location: https://repo.almalinux.org/almalinux/10/cloud/***/images/AlmaLinux-10-GenericCloud-latest.***.qcow2
arch: ***

```

By editing the yaml document, or a PhD in yq, you can make it use the latest daily image.

But maybe it should be a flag to the create command, to go straight for latest-greatest?

* #5032

It might require a `latest: true` metadata?

```go
type Image struct {
File `yaml:",inline"`
Latest *bool `yaml:"latest,omitempty" json:"latest,omitempty"`
Kernel *Kernel `yaml:"kernel,omitempty" json:"kernel,omitempty"`
Initrd *File `yaml:"initrd,omitempty" json:"initrd,omitempty"`
}
```

Start at the start/create command and the Image struct shown in the issue, then trace how the YAML images list is selected. Review #5032 for related context and define how a --latest option and any latest metadata should affect selection; done means the command can choose the latest image without manually editing YAML.

Written by the indexing model from the issue text.