swarmexec v1.1.4 — agent image defaults to Docker Hub

`swarmexec init` now deploys the agent from Docker Hub by default:
docker.io/logleio/swarmexec-agent:latest, a public image, so init needs no
`docker login`. Releases are still mirrored to the GitLab registry, and
SWARMEXEC_AGENT_IMAGE / --image override the source (pin a version or use
the mirror).

Agent detection follows the rename: label-first (swarmexec.role=agent),
with an image-name fallback that now matches both the Docker Hub name and
the legacy GitLab name, so a cluster mid-migration is recognized either
way. The deploy manifests carry the role label too.

No API or wire-protocol change; a running agent stays compatible.