docs(agent-hierarchy): drop missed manager-root mention from apply_set_parent refuses list (argus on #758)

docs/agent-hierarchy.md

@@ -30,11 +30,16 @@ operator-driven (#486 / #487):
 
 All three converge on `topology::set_parent`, which delegates the
 validation rules to a pure `apply_set_parent` helper. Refuses:
-- reparenting the manager (structurally root),
 - unknown `child` / `new_parent` (typo guard),
 - self-parenting,
 - cycles (32-hop ancestor walk, mirroring `is_descendant_of`).
 
+Post-#743 the manager is reparentable like any other agent — the
+"structurally root" carve-out was historical paranoia; the manager's
+privileges live on its MCP socket, not its tree position, and the
+cycle walk above catches the only real safety concern (moving the
+manager under one of its own descendants).
+
 Idempotent no-op fast path skips the disk write when the parent is
 already what's requested. After a successful write the surfaces call
 `Coordinator::rescan_containers_and_emit` so connected dashboard