wip(#3001): power chains name their group roots

The `*_many` entry points returned `insert_job`'s result while their closures
ended in `Vec::new()` — naming nothing, so the returned id list was always
empty. `queued_dags` would have shipped `Some([])` and hivectl's wait loop would
have had nothing to poll. Silent: it compiles, the op still runs, and no test in
isolation looks.

Each `*_chain` now returns its group root's guid and the `*_nodes` collectors
gather them, so the ids a caller gets back are the roots it can actually wait on.

`start_chain` returns *four* in the stale branch, not one: `rebuild_nodes`
chains its roots behind `SetWanted` with `after_ok` rather than nesting them
under it, so `SetWanted` rolls up only itself. Naming it alone would have
reported the start complete while the rebuild was still running — the same
under-reporting bug one level down.

hive-c0re/src/dashboard/lifecycle_ops.rs

@@ -110,7 +110,8 @@ pub(super) async fn post_kill(
     // `socket_server.rs::Request::Kill` stays in place: a
     // manager calling Kill on its own container is self-suicide
     // mid-call, not a legitimate operator action.
-    if let Err(e) = crate::job_queue::power::stop_many(&state.coord, &[logical.clone()], false).await
+    if let Err(e) =
+        crate::job_queue::power::stop_many(&state.coord, &[logical.clone()], false).await
     {
         tracing::error!(agent = %logical, error = ?e, "stop: insert failed");
     }