Creates the decnet system user/group, installs every unit file from
deploy/ into /etc/systemd/system, drops the polkit rule, seeds
/opt/decnet + /var/{lib,log}/decnet + /etc/decnet + /run/decnet,
writes a placeholder /etc/decnet/config.ini, applies the new
tmpfiles.d entry so /run/decnet survives reboots, daemon-reloads,
and `systemctl enable --now decnet.target`.
Idempotent (re-runs print [SKIP] on already-configured items),
--dry-run previews the plan without touching anything, --no-start
defers the target start, --force overwrites even matching unit
files. Master-only (added to MASTER_ONLY_COMMANDS).
9 orchestration tests cover the non-root gate, dry-run, useradd/
groupadd argv, SKIP on present user/group, unit-file idempotency,
--force overwrite, --no-start suppression, happy path, and the
"deploy/ not found" error message.
