feat(#2453): remove DAG parent_id now that every op is one DAG

With the meta-update cascade (#2476) and startup sweep (#2450) folded
into single DAGs that grow per-agent subgraphs via append_subgraph,
nothing links parent/child DAGs anymore — parent_id is dead.

hive-c0re: drop parent_id from Dag/DagSpec (+ the DagView copy); delete
append_children and cancel_children (no callers); simplify trim_history
(no more terminal-parent-with-live-children guard — a one-big-DAG is
terminal only when its whole graph settles); drop the rebuild() parent_id
param; QueueDag returns just the polled DAG (no fan-out children to
gather). hive-sh4re: drop the DagView.parent_id wire field.

frontend: a multi-step op is one DAG now, so renderRebuildQueue drops the
childrenOf/orphans cross-DAG grouping and renders each entry flat; its
per-agent subgraphs render as nodes within the one row (split by deps).
Removed the dead rqe-child style + isChild plumbing.

Docs + the child-DAG queue tests updated/removed to match.

hive-c0re/src/job_queue/submit.rs

@@ -38,10 +38,10 @@ fn submit_and_emit(coord: &Arc<Coordinator>, spec: super::DagSpec) -> u64 {
 }
 
 /// Manual/approval-independent rebuild (always relocks the agent's
-/// meta input — cascade children are built by the scheduler's fan-out
-/// instead of this surface).
+/// meta input — the meta-update cascade grows its own rebuild subgraphs
+/// in-DAG instead of going through this surface).
 pub fn rebuild(coord: &Arc<Coordinator>, agent: &str, source: Source, reason: String) -> u64 {
-    submit_and_emit(coord, templates::rebuild(agent, source, reason, None, true))
+    submit_and_emit(coord, templates::rebuild(agent, source, reason, true))
 }
 
 // ---- dynamic power-op DAG assembly ----------------------------------------
@@ -148,7 +148,6 @@ fn power_dag(
         template,
         source,
         reason,
-        parent_id: None,
         approval_id: None,
         inputs: Vec::new(),
         perm_payload: None,