Add VirtualMachinesMetadataService::buildAgentConfiguration()/getAgentYaml(),
mirroring agent.yaml's structure (nats, agent, iso, log, autoheal) and
including it as an 'agent' key in getMetadata(). NATS endpoints are
pulled from the same events.nats.* config the platform's own
NatsService uses, instead of being hardcoded separately, so the two
can no longer drift apart.

updateConfigurationIso() now renders agent.yaml from this shared
config instead of reading the static agents/vm-service/agent.yaml
template and string-replacing {agent_uuid}/{api_key} placeholders.