Support executing `install to-filesystem` from host directly

As someone who used the workflow of "install to-filesystem" to get a system compatible with bootable containers. I used the following command (successfully):

```
$ sudo yum install -y podman
$ sudo podman run --rm --privileged --pid=host --security-opt label=type:unconfined_t -v /:/target
-v /var/lib/containers:/var/lib/containers quay.io/swalter/monday:1.0 bootc
install to-filesystem --karg=console=ttyS0,115200n8 --replace=alongside /target
$ sudo reboot
```

This command is not discoverable. I would have expected the to use the ```bootc``` command directly, or have a much simpler podman command.

Start by reviewing the existing `bootc install to-filesystem` entry point and the documented container invocation shown in the issue. Define how the operation should be invoked directly from the host or through a simpler Podman command, then verify that it preserves the requested kernel arguments, replacement mode, and target filesystem behavior.

索引モデルが issue の本文から書いたものです。