Add blacksmith-2vcpu-ubuntu-2404 to actionlint self-hosted-runner labels config
to suppress "unknown label" warnings during workflow linting.
This label is used across all workflows after the Blacksmith migration.
Add actionlint.yaml config to declare lxc-ci as a known custom label
for self-hosted runners, fixing the actionlint CI check.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>