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/link/protocol.go

@@ -420,6 +420,20 @@ const LivenessContract = 1
 // because an older one parses strictly and would refuse the whole declaration for it.
 const ReadinessContract = 2
 
+// RootContract is the statement of an engine that also judges a user's declared `root` (novox/hq ADR 0266):
+// whether an account declared never to become root without a person can — uid 0, a group that grants root,
+// a sudo rule, a secret of the mesh it may read. Only to such an engine is the field sent: an older one
+// parses strictly and would refuse the whole declaration for it.
+const RootContract = 3
+
+// ReasonRoot starts the reason of an account verdict that found a way to root (ADR 0266); the node-engine's
+// own words (mesh-host internal/accounts ReasonRoot).
+const ReasonRoot = "can become root without a person"
+
+// RootNever is the value of a user's `root`, and of a verdict's Root, that the account must never become
+// root without a person (ADR 0266).
+const RootNever = "never"
+
 // Health is one statement of a machine's long-running resources (to-be 48 §4): in every report, as the
 // event HealthSubject between reports on each change, and again every minute while one is not healthy.
 // The node-engine's own (mesh-host internal/link Health); a test on each side holds the field names.
@@ -542,6 +556,10 @@ type ResourceHealth struct {
 	// manager, and the account itself for a resource of kind KindAccount (novox/hq ADR 0254). Empty from an
 	// engine older than that, and for anything the machine's own manager or runtime runs.
 	Account string `json:"account,omitempty"`
+	// Root is "never" on a verdict of kind KindAccount whose account is declared never to become root
+	// without a person (novox/hq ADR 0266): the engine judged that too, and a healthy verdict says it cannot.
+	// Empty from an engine older than RootContract, and on every other verdict.
+	Root string `json:"root,omitempty"`
 }
 
 // HealthSaid is the health event's body: the machine and its statement. The machine is read from the