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

hive-c0re/src/coordinator.rs

@@ -509,6 +509,89 @@ impl Coordinator {
         }
     }
 
+    /// Apply a topology reparent + fan the resulting notifications
+    /// out to the three affected agents (#743). On success, drops a
+    /// one-line system message into the inbox of:
+    ///
+    /// 1. The **old parent** (if any) — `"{child} moved out of your
+    ///    subtree to {new_parent_or_root}"`.
+    /// 2. The **new parent** (if any) — `"{child} just moved into
+    ///    your subtree (was previously under
+    ///    {old_parent_or_root})"`.
+    /// 3. The **moved agent** — `"your parent changed from
+    ///    {old_parent_or_root} to {new_parent_or_root}"`.
+    ///
+    /// `_or_root` resolves to the literal string `"<root>"` when the
+    /// slot is `None`, keeping the wording consistent with the
+    /// `<parent>` sentinel's "root → operator" routing (#692). The
+    /// notifications fire as ordinary broker messages with
+    /// `from = hive_sh4re::SYSTEM_SENDER` so the dashboard renders
+    /// them under the existing system-source styling.
+    ///
+    /// Idempotent: if `topology::set_parent` skipped the disk write
+    /// (same-parent no-op), no messages fire. The `topology` module's
+    /// validation (`apply_set_parent`: unknown agent, cycle, etc.)
+    /// runs *before* any message is sent, so a refused move never
+    /// notifies anyone.
+    ///
+    /// Also drives a `rescan_containers_and_emit` after the write so
+    /// the dashboard tree re-renders without polling.
+    pub async fn reparent_with_notify(
+        self: &Arc<Self>,
+        child: &str,
+        new_parent: Option<&str>,
+    ) -> std::result::Result<(), String> {
+        // Snapshot the old parent BEFORE the write so the
+        // notifications can describe both sides of the change.
+        let topo_before = crate::topology::read();
+        let old_parent = topo_before.get(child).cloned().flatten();
+
+        // The disk write happens here; topology validation +
+        // idempotent fast-path inside `set_parent` may short-circuit
+        // (same parent → no-op). We mirror the same idempotent shape
+        // for the notifications: if nothing changed, send nothing.
+        crate::topology::set_parent(child, new_parent)?;
+
+        let changed = old_parent.as_deref() != new_parent;
+        if changed {
+            let old_label = old_parent.as_deref().unwrap_or("<root>");
+            let new_label = new_parent.unwrap_or("<root>");
+            // System-source notifications. Best-effort: a failed broker
+            // send is logged + ignored so a transient sqlite error
+            // doesn't bubble out and unwind the topology write.
+            if let Some(op) = old_parent.as_deref() {
+                let _ = self.broker.send(&hive_sh4re::Message {
+                    from: hive_sh4re::SYSTEM_SENDER.to_owned(),
+                    to: op.to_owned(),
+                    body: format!("{child} moved out of your subtree to {new_label}"),
+                    in_reply_to: None,
+                });
+            }
+            if let Some(np) = new_parent {
+                let _ = self.broker.send(&hive_sh4re::Message {
+                    from: hive_sh4re::SYSTEM_SENDER.to_owned(),
+                    to: np.to_owned(),
+                    body: format!(
+                        "{child} just moved into your subtree (was previously under {old_label})"
+                    ),
+                    in_reply_to: None,
+                });
+            }
+            let _ = self.broker.send(&hive_sh4re::Message {
+                from: hive_sh4re::SYSTEM_SENDER.to_owned(),
+                to: child.to_owned(),
+                body: format!("your parent changed from {old_label} to {new_label}"),
+                in_reply_to: None,
+            });
+        }
+
+        // Rescan + diff-emit regardless of whether messages fired —
+        // even an idempotent no-op might have happened concurrently
+        // with another lifecycle event the dashboard cares about.
+        self.rescan_containers_and_emit().await;
+        Ok(())
+    }
+
     /// Read-only snapshot of the last cached container view. Used by
     /// `/api/state` to cold-load page-open clients without re-running
     /// `nixos-container list` themselves; the

hive-c0re/src/dashboard.rs

@@ -2188,16 +2188,21 @@ async fn post_set_parent(
         .map(str::trim)
         .filter(|s| !s.is_empty())
         .map(str::to_owned);
-    match crate::topology::set_parent(&child, new_parent.as_deref()) {
+    // `reparent_with_notify` wraps `topology::set_parent` with the
+    // three notification messages (#743) + the ContainerView rescan.
+    // Idempotent same-parent calls skip both the messages and the
+    // disk write per the topology fast-path.
+    match state
+        .coord
+        .reparent_with_notify(&child, new_parent.as_deref())
+        .await
+    {
         Ok(()) => {
             tracing::info!(
                 child = %child,
                 new_parent = ?new_parent,
                 "operator: set-parent via dashboard"
             );
-            // Topology drives ContainerView.parent; refresh the
-            // snapshot so connected viewers see the new tree.
-            state.coord.rescan_containers_and_emit().await;
             (StatusCode::OK, "ok").into_response()
         }
         Err(e) => error_response(&format!("set-parent {child} failed: {e}")),

hive-c0re/src/server.rs

@@ -187,13 +187,15 @@ async fn dispatch(req: &HostRequest, coord: Arc<Coordinator>) -> HostResponse {
             }
             HostRequest::SetParent { child, new_parent } => {
                 tracing::info!(%child, ?new_parent, "set_parent");
-                crate::topology::set_parent(child, new_parent.as_deref())
+                // `reparent_with_notify` wraps `topology::set_parent`
+                // with the three notification messages (#743) + the
+                // ContainerView rescan. Idempotent same-parent calls
+                // skip both the messages and the disk write per the
+                // topology fast-path.
+                coord
+                    .reparent_with_notify(child, new_parent.as_deref())
+                    .await
                     .map_err(anyhow::Error::msg)?;
-                // ContainerView.parent is read from topology.json — a
-                // change here means every container row potentially
-                // moves in the dashboard tree. Rescan + diff-emit so
-                // open viewers repaint without polling.
-                coord.rescan_containers_and_emit().await;
                 HostResponse::success()
             }
         })

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]