stabilization tracker: `bootc image copy-to-storage`

Original PR: https://github.com/containers/bootc/pull/620

This issue tracks the stabilization of `bootc image`, which currently just has one subcommand `bootc image copy-to-storage`.

## Rationale

We want to make it ergonomic and convenient to operate on the booted host image via e.g. `podman`. Today usage of podman defaults to operating on `/var/lib/containers`, a distinct mutable store.

Having the ability to conveniently copy the image to the booted storage aids with https://containers.github.io/bootc/booting-local-builds.html

## Concerns

- Longer term, we want to unify bootc and podman storage; but perhaps `copy-to-storage` can become a no-op
- Would it be better to try to instead expose an API to *read* from the bootc storage, and wrap that in a CLI?

Review original PR #620 and the current `bootc image copy-to-storage` entry point first. Compare its behavior with the booting-local-builds documentation and the stated Podman storage goal. Done requires an agreed stabilization direction—whether to retain, replace, or make the command a no-op—plus an implementation scope and validation plan.

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