refactor(job-queue): build DAGs by naming nodes, not counting them

Every template built a `Vec<NodeSpec>` whose edges and parents were
positional indices into that vector, so a shape was expressed as
arithmetic: `base + 1`, `stop_root + 2`, `sfu + 1`, and a
`reconcile_index()` helper that read the emitted vector's length to find
out where its own last node had landed. `concat_subgraphs` existed
solely to rebase one per-agent subgraph's indices onto another's.

Templates now declare into a `hive_jobq::JobBuilder` and hold the
handles they get back, so an edge names the node it waits on. The
arithmetic is gone, and with it:

- `NodeSpec` and the job-queue's own index-based `Dep`.
- `insert_group`'s index resolution — it wraps `Scheduler::insert_job`.
- `concat_subgraphs` — per-agent chains share one builder and each keeps
  its own root, so independence is structural rather than computed.
- `reconcile_index` and `dep_index`.
- `templates::validate` and its petgraph toposort. It rejected dangling
  deps and cycles; both are now unrepresentable, since a handle only
  exists for an already-declared node and every edge therefore points
  backwards. (petgraph stays in the tree for `agent_config::topology`.)

`NodeOutput.append_subgraph` becomes `Vec<Job>`: an executor cannot
reach the queue, so it hands back declarations and the scheduler inserts
them under its own lock. That is what the in-DAG growth path always
wanted — a transferable declaration, not a vector of specs.

Resource declaration is unchanged in behaviour: the `templates::node`
helper applies `NodeKind::resource_deps()` at the construction site, so
every node still declares what its kind needs. Moving that declaration
to the call sites is #2818's job; this leaves it one place to delete.

Three tests went with the guard they covered — they hand-built malformed
specs out of indices, which is the representation that made those shapes
possible. Two more now read a DAG's shape off the queue rather than out
of a spec vector, which is where it is observable. The remaining 45
job-queue tests are unchanged and still pass: lease serialization,
roll-up, cancel-cascade, in-DAG growth and per-agent concurrency all
behave as before.

hive-c0re/src/job_queue/scheduler.rs

@@ -125,7 +125,7 @@ fn handle_completion(coord: &Arc<Coordinator>, done: NodeDone) {
             // `Done` just below — covers both the multi-node case (a `MetaLock`
             // growing per-agent rebuild subgraphs) and the single-node case (a
             // `Reconcile` planner's `Start` / `Stop`).
-            for subgraph in &output.append_subgraph {
+            for subgraph in output.append_subgraph {
                 coord
                     .job_queue
                     .append_subgraph(claim.dag_id, subgraph, claim.node_id);