swarm-archive serves restic repositories with rclone, append-only, one login
and one repository per agent. swarm-controller writes each placed agent's
login and restic password to the store, replaces them on a move and removes
them when the agent is destroyed. Every hive backs its agents up hourly under
its own store identity. The archive host prunes by retention.
hive-priv copies a stopped container's root to the agent's work dir and
renames it aside before passing nspawn --ephemeral; the agent's nix state
moves to the persistent profiles bind. Adds the agent-ephemeral-rootfs vm
test and a module-eval case for the nix state redirect.
