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/declaration.go

@@ -241,6 +241,31 @@ type Rendering struct {
 	// refuses a field it does not know, whole — so to it the field is not sent, and what it runs is
 	// judged by liveness alone.
 	ReadsHealth bool
+
+	// JudgesRoot says this machine's node-engine judges a user's declared `root` (novox/hq ADR 0266: its
+	// statement's contract is link.RootContract or later). To an older, strict engine the field is not
+	// sent, and the account it names is not judged — which the self-check says, as not judged.
+	JudgesRoot bool
+}
+
+// RootField is a user resource's field saying the account must never become root without a person
+// (novox/hq ADR 0266).
+const RootField = "root"
+
+// rootInto composes a user's `root` for the node-engine: taken away when it asserts nothing (empty — a
+// machine where agents run as the operator) or when the engine is older than the field and parses
+// strictly; kept as "never" otherwise.
+func rootInto(resource map[string]any, with Rendering) {
+	if resource["type"] != "user" {
+		return
+	}
+	value, has := resource[RootField]
+	if !has {
+		return
+	}
+	if s, _ := value.(string); s == "" || !with.JudgesRoot {
+		delete(resource, RootField)
+	}
 }
 
 // machinePort is where a module's port lives on this machine, or the port itself when the mesh has
@@ -981,6 +1006,9 @@ func (r Resolution) compose(with Rendering, owner map[string]string,
 			// How it is ready, in the node-engine's words: its endpoint as the port this machine
 			// published it on — or not sent at all to an engine older than the field (ADR 0240).
 			healthInto(copied, m, with)
+			// And a user's `root` (novox/hq ADR 0266): sent only when it asserts something, to an engine
+			// that judges it.
+			rootInto(copied, with)
 			// The account's environment and every module's shell code, where this module holds the
 			// seat that places them (novox/hq ADR 0203, ADR 0204). Gathered from every module on
 			// the node, as the jails are, and **last of every placeholder pass**: shell code is a