topology: drop manager-root special case + notify three agents on reparent (#743)

hive-c0re/src/topology.rs

@@ -182,18 +182,23 @@ pub fn default_seed(agent_names: &[String]) -> BTreeMap<String, Option<String>>
 }
 
 /// Pure validation + apply for [`set_parent`]. Splits off so tests
-/// can exercise the rules (cycle / unknown / manager-protect) on
-/// an in-memory `BTreeMap` without touching the on-disk
-/// `topology.json`. Returns either the post-move map (caller
-/// writes it back) or a user-readable error string.
+/// can exercise the rules (cycle / unknown) on an in-memory
+/// `BTreeMap` without touching the on-disk `topology.json`. Returns
+/// either the post-move map (caller writes it back) or a
+/// user-readable error string.
+///
+/// Pre-#743 this also refused to reparent the manager
+/// ("cannot reparent the manager — it is structurally root") —
+/// argus-paranoia from #361 that we dropped per mara's
+/// `#9512` / `#9557`: the manager's special powers come from its
+/// privileged MCP socket, not its tree position. The cycle walk
+/// below covers "moving X under its own descendant" for the
+/// manager as much as any other agent.
 pub fn apply_set_parent(
     topo: &BTreeMap<String, Option<String>>,
     child: &str,
     new_parent: Option<&str>,
 ) -> Result<BTreeMap<String, Option<String>>, String> {
-    if child == crate::lifecycle::MANAGER_NAME {
-        return Err("cannot reparent the manager — it is structurally root".to_owned());
-    }
     if !topo.contains_key(child) {
         return Err(format!("unknown agent: {child}"));
     }
@@ -342,10 +347,34 @@ mod tests {
     }
 
     #[test]
-    fn apply_set_parent_refuses_manager_move() {
-        let err = apply_set_parent(&topo_three_level(), crate::lifecycle::MANAGER_NAME, None)
+    fn apply_set_parent_allows_manager_move() {
+        // Post-#743: the manager is reparentable like any other agent
+        // (its privileges live on the MCP socket, not its tree
+        // position). Build a topo with an unrelated root-level agent
+        // `peer` so moving the manager under it doesn't trip the
+        // cycle walk (every non-manager agent in topo_three_level
+        // descends from the manager, so that fixture can't exercise
+        // a legal manager move).
+        let mut topo = BTreeMap::new();
+        topo.insert(crate::lifecycle::MANAGER_NAME.to_owned(), None);
+        topo.insert("peer".to_owned(), None);
+        let next = apply_set_parent(&topo, crate::lifecycle::MANAGER_NAME, Some("peer"))
+            .expect("manager move should succeed post-#743");
+        assert_eq!(
+            next.get(crate::lifecycle::MANAGER_NAME),
+            Some(&Some("peer".to_owned()))
+        );
+    }
+
+    #[test]
+    fn apply_set_parent_refuses_manager_under_own_descendant() {
+        // Moving the manager under `bob` (who already lives under
+        // `alice` who lives under the manager) would close the loop.
+        // The general cycle walk catches this; no separate manager
+        // guard needed.
+        let err = apply_set_parent(&topo_three_level(), crate::lifecycle::MANAGER_NAME, Some("bob"))
             .unwrap_err();
-        assert!(err.contains("manager"), "err = {err}");
+        assert!(err.contains("cycle"), "err = {err}");
     }
 
     #[test]