The orchestrator spawned agents only by bare image names and built any
missing image from source on the host, so a deployment had to carry the
build context and a toolchain — there was no way to just pull and run the
images the release workflow publishes.
Add two settings (default empty = unchanged local-build behavior):
ROBOCO_AGENT_IMAGE_REGISTRY and ROBOCO_AGENT_IMAGE_TAG. When a registry is
set, the orchestrator spawns and ensures {registry}/roboco-agent-*[:tag] and
pulls (never builds) any image it lacks. Also adds the previously-missing
agent-secretary image to the lazy-build map.
Ship docker-compose.registry.yml: a standalone compose that pulls every
published image (GHCR or Docker Hub, pinnable version) and wires the
orchestrator to spawn the matching pre-built agent images. The existing
build compose files are unchanged.
