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/submit.rs

@@ -24,7 +24,7 @@
 
 use std::sync::Arc;
 
-use super::model::{DagSpec, Dep, NodeKind, NodeSpec, Template};
+use super::model::{DagSpec, Dep, NodeKind, NodeSpec};
 use super::templates::{after_ok, child, node, rebuild_nodes};
 use super::{Source, templates};
 use crate::coordinator::{Coordinator, TransientKind};
@@ -188,16 +188,17 @@ fn concat_subgraphs(chains: Vec<Vec<NodeSpec>>) -> Vec<NodeSpec> {
     out
 }
 
-/// Wrap assembled power-op `nodes` in a `DagSpec`.
+/// Wrap assembled power-op `nodes` in a `DagSpec`. No terminal hook: a power
+/// op's effect is its nodes (`SetWanted` + `Reconcile`), with nothing left to
+/// do once they settle.
 fn power_dag(
-    template: Template,
     transient: TransientKind,
     source: Source,
     reason: String,
     nodes: Vec<NodeSpec>,
 ) -> DagSpec {
     DagSpec {
-        template,
+        hook: None,
         source,
         reason,
         approval_id: None,
@@ -223,13 +224,7 @@ pub(crate) fn stop_spec(
         .iter()
         .map(|(agent, running)| stop_chain(agent, graceful, *running))
         .collect();
-    let template = if graceful {
-        Template::GracefulStop
-    } else {
-        Template::Stop
-    };
     power_dag(
-        template,
         TransientKind::Stopping,
         source,
         reason,
@@ -256,13 +251,7 @@ pub(crate) fn start_spec(
     } else {
         TransientKind::Starting
     };
-    power_dag(
-        Template::Start,
-        transient,
-        source,
-        reason,
-        concat_subgraphs(chains),
-    )
+    power_dag(transient, source, reason, concat_subgraphs(chains))
 }
 
 /// Assemble the restart DAG from explicit `(agent, running)` targets.
@@ -276,13 +265,7 @@ pub(crate) fn restart_spec(
         .iter()
         .map(|(agent, running)| restart_chain(agent, graceful, *running))
         .collect();
-    let template = if graceful {
-        Template::GracefulRestart
-    } else {
-        Template::Restart
-    };
     power_dag(
-        template,
         TransientKind::Restarting,
         source,
         reason,