broker: resolve <parent> sentinel to topology parent at send time (#692)

hive-sh4re/src/lib.rs

@@ -598,6 +598,15 @@ pub const MANAGER_AGENT: &str = "manager";
 /// dashboard's inbox view — they are never `recv`'d by an agent harness.
 pub const OPERATOR_RECIPIENT: &str = "operator";
 
+/// Reserved magic recipient — `send(to: "<parent>", ...)` is rewritten
+/// by hive-c0re at delivery time to whoever `topology::parent_of(sender)`
+/// returns, or to [`OPERATOR_RECIPIENT`] when the sender is a root agent
+/// (no parent). Lets agents address their parent without hardcoding the
+/// label, so runtime reparenting (#486) requires no agent-side restart.
+/// The angle brackets are not valid in agent names (validators reject
+/// `<`/`>`), so this name can never collide with a real recipient.
+pub const PARENT_RECIPIENT: &str = "<parent>";
+
 /// 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";