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.

internal/catalogue/runtime.go

@@ -83,6 +83,11 @@ const (
 	RuntimeBrokerFile      = "MESH_BROKER_FILE"
 	RuntimeOperatorAccount = "MESH_OPERATOR_ACCOUNT"
 	RuntimeOperatorHome    = "MESH_OPERATOR_HOME"
+	// RuntimeAgentAccount and RuntimeAgentHome are the account agents run as on the machine and its home
+	// (novox/hq ADR 0266): the agent account where the node names one, the operator account otherwise.
+	// The agent's module writes the agent's home from them; absent where neither account is known.
+	RuntimeAgentAccount = "MESH_AGENT_ACCOUNT"
+	RuntimeAgentHome    = "MESH_AGENT_HOME"
 	// RuntimeToolEnv is every served module's composed environment, as JSON (novox/hq ADR 0192):
 	// {"<module>": {"<word>": "<value>"}}. The runtime takes it at start, removes it from its own
 	// environment and hands each module's words to that module's bundles alone. In the unit, so a
@@ -215,6 +220,10 @@ func (r Resolution) runtimeProcess(with Rendering) (map[string]any, error) {
 		env[RuntimeOperatorHome] = accountHomeOf(r.Account, r.AccountHome)
 		process["user"] = r.Account
 	}
+	if agent, home := r.agentAccount(); agent != "" {
+		env[RuntimeAgentAccount] = agent
+		env[RuntimeAgentHome] = home
+	}
 	// Routed through the artifact store as this network reaches it now, like everything the mesh
 	// built; refused with the same words when there is no store to route through.
 	if err := artifactsInto(process, RuntimeModule, with); err != nil {