Let a user say it never becomes root, so the agents' account can be judged

A declaration may now state root: never on a user (novox/hq ADR 0266), and a
health statement carries it under contract 3; the judging follows.

internal/declaration/declaration.go

@@ -383,8 +383,25 @@ type User struct {
 	// has it not. On the account rather than on the unit, because it is the account's: two units of
 	// one account cannot disagree about it, and undeclaring one of them must not stop the other.
 	Linger *bool `json:"linger,omitempty"`
+
+	// Root says whether this account may become root without a person (novox/hq ADR 0266). "never"
+	// is the agents' own account: the login an agent session runs as on a machine where it must not
+	// reach root by itself. Empty asserts nothing, as Shell's does.
+	//
+	// **A statement the engine judges, never one it acts on.** The apply gives an account it creates
+	// no password, no sudo rule and no group beyond those declared, as it always has, and takes none
+	// away from one it finds: a sudo rule or a group granted by hand is a person's to remove, and a
+	// declaration that silently stripped them would be the mesh deciding what a person's machine
+	// grants. What "never" adds is the look: on every look the engine reads whether the account can
+	// become root by itself — by its uid, a group that grants root, any sudo rule, or a secret the mesh
+	// placed that it can read — and says it unhealthy while it can (internal/accounts), so the
+	// controller can tell a machine where it holds from one where it does not.
+	Root string `json:"root,omitempty"`
 }
 
+// RootNever is the one value Root takes besides empty: the account never becomes root without a person.
+const RootNever = "never"
+
 // Network is a named network on this machine.
 //
 // **A name and nothing else.** Not a driver, a subnet or a gateway: each of those is something a
@@ -478,6 +495,9 @@ func (u *User) validate(where string, _ bool) []string {
 	if u.Home != "" && !strings.HasPrefix(u.Home, "/") {
 		problems = append(problems, where+": a home directory is an absolute path")
 	}
+	if u.Root != "" && u.Root != RootNever {
+		problems = append(problems, fmt.Sprintf("%s: root is %q or absent, and %q is neither", where, RootNever, u.Root))
+	}
 	return problems
 }