refactor(#2908): cancel a node, not a DAG

`JobQueue::cancel(dag_id)` resolved the id to a `NodeKind::Dag` container
and cancelled that. But the container lookup was the only DAG-specific
part — everything that makes cancel work already lives in the scheduler:
`cancel_node` marks the node `Cancelled` and cascades to its pending
descendants, sparing any node whose edge accepts `Cancelled` (which is
what keeps a dropped approval DAG from dangling its row).

So `cancel` now takes any node id. A group root cancels the whole group,
which is what the dashboard's button does today and why nothing about
its behaviour changes: a DAG id *is* its root node's id. An interior
node cancels just that branch — a capability the DAG-scoped version
could not express, covered by the new test (a hive-wide restart drops
one agent's subgraph while the other keeps running).

`QueueInner::node_by_id` replaces `container()` here: same search, same
cost, without asserting the node is a DAG container. `container()` stays
for `first_error` and the append-subgraph guard, which are genuinely
DAG-scoped.

No wire change. The route is `POST /api/rebuild-queue/{id}/cancel` with
a `u64` path param — same type, same route, and the client keeps sending
the same number. Only the param's documented meaning moves from "DAG id"
to "node id".

Checked with clippy (`--all-targets -D warnings`), `cargo test -p
hive-c0re` (322 passed) and `nix fmt`. No option surface touched, so no
nix-eval gate.

hive-c0re/src/job_queue/tests.rs

@@ -1010,6 +1010,38 @@ fn cancel_clears_queued_dag() {
     assert_eq!(state_of(&q, id), State::Cancelled);
 }
 
+/// `cancel` takes a **node** id, not a DAG id — so an interior node can be
+/// dropped without touching the rest of the group.
+///
+/// This is the capability the DAG-scoped version couldn't express, and the
+/// reason it reads naturally: a DAG id *is* its root node's id, so the
+/// whole-group cancel every other test does is just this called on a root.
+/// Here a hive-wide restart drops **one agent's** subgraph and the other agent
+/// still runs.
+#[test]
+fn cancel_drops_one_agents_branch_leaving_the_rest() {
+    let q = JobQueue::new(2);
+    let id = submit(&q, restart_online(&["agent-a", "agent-b"], false, "r"));
+    // Per-agent subgraphs are independent roots; find agent-a's.
+    let snap = q.snapshot();
+    let dag = snap.iter().find(|d| d.id == id).expect("dag in snapshot");
+    let a_root = dag
+        .nodes
+        .iter()
+        .find(|n| n.agent == "agent-a" && n.parent.is_none())
+        .expect("agent-a has a group root");
+
+    assert!(q.cancel(a_root.id), "an interior/group root cancels alone");
+
+    // agent-b's work is untouched and still claimable; agent-a's is not.
+    let claims = q.claim_ready();
+    assert!(
+        !claims.is_empty() && claims.iter().all(|c| c.agent == "agent-b"),
+        "only agent-b remains runnable, got {:?}",
+        claims.iter().map(|c| c.agent.as_str()).collect::<Vec<_>>()
+    );
+}
+
 #[test]
 fn cancel_refuses_running_dag() {
     let q = JobQueue::new(1);