feat(guests): let a guest nest OCI containers (task 0009)

Add a `nesting` placement field to the Host-side guest interface, a bool
off by default. On, it grants the guest's container the prerequisites its
interior needs to run Podman and other OCI containers: the `CAP_NET_ADMIN`
capability an OCI runtime uses to build its bridges and firewall rules,
and the `/dev/net/tun` and `/dev/fuse` device nodes it reaches for to
network those containers and back their overlay storage. Off, both the
capability and device lists are empty, so a non-nesting guest is untouched.

cgroup delegation, the other nested prerequisite, the NixOS container
backend already grants every container unconditionally, so the Skeleton
records it with an absence pointer rather than re-emitting it.

Add a nesting-sample guest whose interior defines an `oci-containers`
workload on Podman, and enable it on neogaia with `nesting` on, so the
path builds end to end through the Host's `nix flake check` — which pulls
in podman and the generated container unit for the nested system.

guests/nesting-sample.nix

@@ -0,0 +1,11 @@
+args@{ my, ... }:
+# A sample guest whose interior runs an OCI container on Podman.
+# The image is pulled at runtime, so the guest builds with no build-time fetch.
+my.guest {
+  name = "nesting-sample";
+  interior = {
+    virtualisation.oci-containers.containers.hello = {
+      image = "docker.io/library/hello-world";
+    };
+  };
+} args