wip(#3001): delete NodeKind::Dag, the container this issue is about

The variant, its label, its agent-accessor arm and its no-op executor arm are
gone, along with the module prose describing a job as "a single container node
whose subtree is the work". A job is now just its nodes: a template declares
them and names the roots it wants back.

`dag_of` becomes `root_of`. It always wrapped the graph's `root_of` and still
returns the same thing, but the old name asserted a concept that no longer
exists — with no container, the parent chain ends at whichever root the template
declared, so the honest question is "which root owns this node", not "which DAG
is this in".

One comment kept its old wording on purpose: `visible_roots` explains that the
projection it replaced keyed on the container kind rather than selecting
structurally. That is a statement about the past and stays true; it now says
"the since-removed container kind" rather than naming a type that is not there
to look up.

hive-c0re/src/job_queue/scheduler.rs

@@ -93,7 +93,7 @@ pub async fn run_worker(coord: Arc<Coordinator>) {
                 let coord = node_coord;
                 async move {
                     tracing::info!(
-                        dag = coord.job_queue.dag_of(id).unwrap_or_default(),
+                        dag = coord.job_queue.root_of(id).unwrap_or_default(),
                         node = id.get(),
                         kind = kind.as_str(),
                         agent = %kind.agent(),