feat(#704): add <children> recipient sentinel — fan-out to direct descendants

hive-sh4re/src/lib.rs

@@ -489,6 +489,14 @@ pub const OPERATOR_RECIPIENT: &str = "operator";
 /// `<`/`>`), so this name can never collide with a real recipient.
 pub const PARENT_RECIPIENT: &str = "<parent>";
 
+/// Reserved magic recipient — `send(to: "<children>", ...)` fans out to
+/// every agent whose direct parent (per `topology.json`) is the sender.
+/// Lets a sub-manager nudge its subtree without enumerating labels at
+/// call-time; topology changes propagate for free. The angle brackets
+/// are structurally safe — agent name validation rejects `<`/`>`.
+/// Delivers to an empty set (no-op) for leaf agents that have no children.
+pub const CHILDREN_RECIPIENT: &str = "<children>";
+
 /// Sender hive-c0re uses for events it pushes into the manager's inbox.
 /// Manager harness recognises this and parses the body as a `HelperEvent`.
 pub const SYSTEM_SENDER: &str = "system";