fleeting-plugin-libvirt v0.1.1

Tests and documentation; no change to plugin behaviour or configuration.

- Ships fleeting's provider integration test (`integration.TestProvisioning`). It is skipped unless `FLEETING_LIBVIRT_BASE_VOLUME` is set; run it with `just integration` (see CONTRIBUTING.md). A failed run removes the instances it left under the name prefix.
- README: the docker-autoscaler executor needs a docker CLI in the guest image (`podman-docker` on a podman guest).
- SECURITY.md and README: VF `spoofchk` stops MAC spoofing only; ARP and IP protection needs a libvirt network interface with `nwfilter`.
- Releases are now also published on the GitHub mirror.

Install: `registry.gitlab.com/dunn.dev/fleeting-plugin-libvirt:0.1.1` (aliases `0.1`, `0`, `latest`).

Verify:

```
cosign verify \
  --certificate-identity-regexp '^https://gitlab\.com/dunn\.dev/fleeting-plugin-libvirt//\.gitlab-ci\.yml@refs/tags/.+$' \
  --certificate-oidc-issuer https://gitlab.com \
  registry.gitlab.com/dunn.dev/fleeting-plugin-libvirt:0.1.1
```

Before 1.0, a minor release (0.2) may rename, remove or change a `plugin_config` key; a patch release (0.1.x) never does. Each break is listed in that release's notes with the change to make.