refactor(#2756): declare the terminal hook instead of inferring it

`Template` was a DAG-level enum that three different things read back
out: `terminal_hook()` mapped it to a side effect, the retention pass
bucketed history by it, and a tracing field printed it. None of those
needed a *label* — they needed the two facts the label happened to
encode. So the enum was a lossy stand-in for intent, and every new DAG
shape had to pick the variant whose inferred behaviour matched, whether
or not the name fit (`reparent` rode `MetaUpdate` for exactly this
reason, with a 10-line comment apologising for it).

Replace the inference with a declaration: `DagSpec.hook:
Option<HookKind>`. Only the builder assembling a DAG knows why it did
so, so only the builder can say what should happen when it settles.
`run_terminal_hook` becomes a field read, and `reparent`'s apology
becomes `hook: None`.

Hook assignment is byte-identical to the old precedence rule
(`approval_id.is_some()` wins, then `Rebuild | PermChange`), checked
site by site; `meta_update` is the only builder with a variable
approval id and so the only remaining conditional.

Retention loses the per-template bucket with the enum that keyed it.
The dashboard renders one recent-builds list, so one flat newest-first
cap (`MAX_HISTORY_DAGS`) bounds it. `HISTORY_GRACE_SECS` goes too — it
existed to stop a burst of same-template DAGs evicting each other
inside one poll interval, which is not a failure mode a flat cap has.
That takes `snapshot_capped()` and the `snapshot_no_grace()` test hook
with it.

The queue is runtime-only (empty graph on boot), so the serde changes
carry no migration risk.

hive-c0re/src/job_queue/scheduler.rs

@@ -53,7 +53,6 @@ pub async fn run_worker(coord: Arc<Coordinator>) {
                     node = claim.node_id.get(),
                     kind = claim.kind.as_str(),
                     agent = %claim.agent,
-                    template = claim.template.as_str(),
                     "job_queue: node running"
                 );
                 let coord = Arc::clone(&coord);