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/workers/auto_update.rs

@@ -303,7 +303,7 @@ fn submit_boot_tree(
     n_deferred: usize,
     n_skipped: usize,
 ) {
-    use crate::job_queue::{DagSpec, NodeKind, NodeSpec, Source, Template};
+    use crate::job_queue::{DagSpec, NodeKind, NodeSpec, Source};
 
     // Fully-quiet boot (nothing stale, nothing drifted) submits nothing.
     if !any_stale && drifted.is_empty() {
@@ -343,7 +343,9 @@ fn submit_boot_tree(
     }
 
     let spec = DagSpec {
-        template: Template::Boot,
+        // The sweep's own rebuild subgraphs emit their `Rebuilt` events as they
+        // land; the boot DAG as a whole has no terminal side effect.
+        hook: None,
         source: Source::AutoUpdate,
         reason,
         approval_id: None,