Name the account agents run as on a node, and say whether it can become root

On the control node every agent ran as the operator's account, which has
passwordless sudo, so an agent could become root without a person (hq ADR
0266). A node now names an agent account at the controller's terminal only;
the agent's module declares it never to become root, the node-engine judges
that, and the self-check (DA) raises agent-can-become-root while it does not
hold, so ADR 0259's router can rest on it.

cmd/mesh-controller/nodes.go

@@ -100,6 +100,12 @@ func nodeCommand(ctx context.Context, args []string) error {
 			"containers reaching outward. The machine reports which of its links face outside; see " +
 			"`node show <name>`")
 
+	case "agent-account":
+		// The account agents run as on this machine, when it is not the operator's (novox/hq ADR 0266). Here,
+		// at the controller's terminal, and nowhere else: no verb and no setting names it, so no agent can
+		// name itself another account.
+		return nodeAgentAccount(ctx, inv, args[1:])
+
 	case "account":
 		// The operator's login on this machine (novox/hq to-be 29): what a home-scoped file is
 		// owned by and which account `ssh <node>` uses. Reports with no argument; sets with one;
@@ -107,7 +113,7 @@ func nodeCommand(ctx context.Context, args []string) error {
 		return nodeAccount(ctx, inv, args[1:])
 
 	default:
-		return fmt.Errorf("node has no %q; it has add, list, show, public-domain and account", args[0])
+		return fmt.Errorf("node has no %q; it has add, list, show, public-domain, account and agent-account", args[0])
 	}
 }
 
@@ -178,6 +184,57 @@ func nodeAccount(ctx context.Context, inv *inventory.Inventory, positionals []st
 	return nil
 }
 
+const agentAccountUsage = "node agent-account <name> — what it is now; " +
+	"<name> <account> [home] to name the account agents run as (home defaults to /home/<account>); " +
+	"<name> --clear to have them run as the operator account again"
+
+// nodeAgentAccount reports, names or clears the account agents run as on a node (novox/hq ADR 0266). Read-
+// shaped with no account, like public-domain; clearing is asked for by name.
+func nodeAgentAccount(ctx context.Context, inv *inventory.Inventory, args []string) error {
+	set := flag.NewFlagSet("node agent-account", flag.ContinueOnError)
+	clear := set.Bool("clear", false, "agents run as the operator account again")
+	positionals, err := parseAround(set, args)
+	if err != nil {
+		return err
+	}
+	if len(positionals) == 0 || len(positionals) > 3 {
+		return errors.New(agentAccountUsage)
+	}
+	node := positionals[0]
+	switch {
+	case *clear && len(positionals) > 1:
+		return fmt.Errorf("name an agent account for %s or --clear, not both", node)
+	case *clear:
+		if err := inv.SetAgentAccount(ctx, node, "", ""); err != nil {
+			return err
+		}
+		fmt.Printf("agents on %s run as the operator account again\n", node)
+		fmt.Printf("  run `push %s` to send it; the agent account itself is kept (the mesh never deletes a login)\n", node)
+		return nil
+	case len(positionals) >= 2:
+		home := ""
+		if len(positionals) == 3 {
+			home = positionals[2]
+		}
+		if err := inv.SetAgentAccount(ctx, node, positionals[1], home); err != nil {
+			return err
+		}
+		fmt.Printf("agents on %s run as %s\n", node, positionals[1])
+		fmt.Printf("  run `push %s` to send it; the self-check says once its node-engine has judged it "+
+			"unable to become root\n", node)
+		return nil
+	default:
+		n, err := inv.NodeByName(ctx, node)
+		if err != nil {
+			return err
+		}
+		for _, line := range agentAccountLines(ctx, inv, n) {
+			fmt.Println(strings.TrimPrefix(line, "  "))
+		}
+		return nil
+	}
+}
+
 const publicDomainUsage = "node public-domain <name> — what it is now; " +
 	"<name> <domain> to set it; <name> --clear to take it away"
 
@@ -592,6 +649,10 @@ func showNode(ctx context.Context, inv *inventory.Inventory, name string) error
 	if err := showMode(ctx, inv, node); err != nil {
 		return err
 	}
+	// Whom agents run as here, and whether that account can become root without a person (ADR 0266).
+	for _, line := range agentAccountLines(ctx, inv, node) {
+		fmt.Println(line)
+	}
 
 	// The domain its routed names are composed under, when it has one (novox/hq ADR 0066). Shown
 	// only when set: a machine that serves nothing to the outside has no domain, and saying so of