feat(agent): run node exporter in nspawn

[hbc (bcho)]

Summary

• resolve node exporter runtime goal state and online artifact source
• install and run node exporter inside the nspawn machine
• add archive verification, TLS configuration, readiness, and preflight integration
• add unit and golden tests

Stack

• PR 2 of 3
• Depends on feat(agent): add node exporter foundation and configuration model #816
• Next: feat(agent): support offline node exporter artifacts #818

Testing

• go test ./api/machina/v1alpha3 ./cmd/agent/internal/daemon ./internal/agentartifacts ./internal/provision ./pkg/agent/...