A user, bytes, and an archive — because most of what people install is

not a service

A shell, a terminal, a chat client, a desktop are a package plus
configuration in somebody's home. A mesh with no notion of a user can own
/etc and nothing anybody looks at, which is most of the reason to manage
a machine at all.

Three shapes, and the vocabulary test asserts the count precisely because
widening it widens what a compromised control plane can express:

  user     a login, its shell and its groups
  archive  a set of files, fetched by digest and unpacked
  (file)   gains `bytes` for what is not text, and `owner`

`user` also makes "zsh is my login shell" declared state. chsh is a
command, the link may not carry one, and a shell settable only by hand is
a shell the mesh cannot manage.

Groups are additive and never pruned — usermod without --append REPLACES
them, which would silently remove every group that makes a login able to
use the machine. A machine's own groups are not the mesh's to know about.

The archive is the one place this host reaches out on its own; everywhere
else it holds one outbound connection and fetches nothing. So it carries
the discipline the bootstrap already uses for images: pinned by digest,
and the digest checked before a single file is written.

Two decisions in the unpacker worth naming:

- an entry naming a path outside the archive is REFUSED, not sanitised.
  Rewriting it to land inside would put a file somewhere nobody asked for
  and report success. Found by the test: the first version quietly
  relocated it.
- symlinks and device nodes are refused rather than skipped, or an
  archive that needed one arrives silently incomplete.

A partial host does archives and refuses users: an archive needs a
filesystem and a way to fetch; a user needs a user database it is allowed
to write.

internal/system/arch.go

@@ -142,3 +142,38 @@ func (arch) SetServiceBoot(ctx context.Context, run Runner, unit, boot string) e
 	_, err := run(ctx, "systemctl", verb, unit)
 	return err
 }
+
+// CreateUser makes a login with useradd.
+//
+// `--create-home` because a user whose home does not exist is a user nothing can be delivered
+// to, and delivering a shell's configuration is most of why the mesh knows about users at all.
+func (arch) CreateUser(ctx context.Context, run Runner, name, home, shell string) error {
+	args := []string{"--create-home"}
+	if home != "" {
+		args = append(args, "--home-dir", home)
+	}
+	if shell != "" {
+		args = append(args, "--shell", shell)
+	}
+	if _, err := run(ctx, "useradd", append(args, name)...); err != nil {
+		return fmt.Errorf("cannot create the user %q: %w", name, err)
+	}
+	return nil
+}
+
+func (arch) SetUserShell(ctx context.Context, run Runner, name, shell string) error {
+	if _, err := run(ctx, "usermod", "--shell", shell, name); err != nil {
+		return fmt.Errorf("cannot set %q's shell to %q: %w", name, shell, err)
+	}
+	return nil
+}
+
+// AddUserToGroup appends, and `--append` is the whole point: without it usermod REPLACES the
+// user's supplementary groups, so a declaration naming one group would silently remove every
+// other — including the ones that make a login able to use a machine at all.
+func (arch) AddUserToGroup(ctx context.Context, run Runner, name, group string) error {
+	if _, err := run(ctx, "usermod", "--append", "--groups", group, name); err != nil {
+		return fmt.Errorf("cannot put %q in the group %q: %w", name, group, err)
+	}
+	return nil
+}