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/doctor.go

@@ -121,6 +121,11 @@ var probeRegistry = []probe{
 	{ID: probeReconnectsID, Asserts: "no user of the bus had its connection dropped more than twelve times in the " +
 		"last hour: the bus module's nats_closed_connections", From: "issue 327", Kind: kindBusReconnects,
 		Phase: 1, run: probeReconnects},
+	// The account agents run as (novox/hq ADR 0266): where a machine names one, its node-engine has judged it
+	// unable to become root without a person — what ADR 0259 §8 rests an authorised answer on.
+	{ID: agentAccountProbe, Asserts: "every machine that names an agent account has it judged, on its node-engine's " +
+		"newest statement, unable to become root without a person", From: "ADR 0266, ADR 0259 §8",
+		Kind: kindAgentCanBecomeRoot, Phase: 1, run: probeAgentAccounts},
 	{ID: "DW", Asserts: "the watchdogs of the signals table ran within three of their intervals",
 		From: "ADR 0227 rule 6: the watchers are watched", Kind: "watchdogs-silent", Phase: 1, run: probeWatchdogs},
 	// The core's health definitions (novox/hq to-be 45 §8, ADR 0236): what a core component's new build is