wip(#3001): convert tests off the container id; drop Source + insert_group

The last of the DAG-container removal. `tests.rs` navigated by the id
`submit` returned, so removing the container removed the tests' way of
finding what they inserted; they name the roots they assert on now, which
is the same handle production uses.

Three findings the port surfaced, each a behaviour change rather than a
test fix:

- Cancelling a rebuild's head no longer drops the job. `Reconcile`'s edge
  accepts a skipped brace, and a cancel-cascade skips rather than cancels,
  so the tail stays claimable. Dropping a job means cancelling every id the
  insert returned.
- A directly-cancelled group root reads terminal while a spared tail still
  runs; the cancel used to land on a node above it, which rolled up
  Finishing instead.
- "One DAG per hive-wide op" is not expressible without a container. The
  three tests asserting it now assert that every named root is top-level,
  which is what makes the per-agent subgraphs concurrent.

Deletes two tests: one asserted only that two containers get distinct ids,
the other re-ran an existing case under a second name.

`Source`, `insert_group` and the stop path's `reason` string went dead with
the container and are removed with it.

hive-c0re/src/server.rs

@@ -840,16 +840,10 @@ async fn handle_stop(
     let mut errors: Vec<String> = Vec::new();
     let mut queued: Vec<u64> = Vec::new();
 
-    // One DAG for all targeted agents — a per-agent stop subgraph each
+    // One insert for all targeted agents — a per-agent stop subgraph each
     // (`SetWanted(Offline) → [Signal → Drain →] Reconcile`), independent
-    // roots that run concurrently on their own leases. A hive-wide
-    // `hivectl stop` is now a single DAG, not N.
+    // roots that run concurrently on their own leases.
     if !agents.is_empty() {
-        let reason = if graceful {
-            "manual via hivectl graceful stop"
-        } else {
-            "manual via hivectl stop"
-        };
         match crate::job_queue::power::stop_many(coord, agents, graceful).await {
             Ok(ids) => {
                 queued.extend(ids.into_iter().map(hive_jobq::NodeId::get));