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/bin/hivectl/dag_progress.rs

@@ -349,7 +349,6 @@ mod tests {
             kind: Template::Rebuild,
             state: State::Running,
             source: Source::Manual,
-            parent_id: None,
             reason: "manual".to_owned(),
             enqueued_at: 0,
             started_at: Some(1),
@@ -389,7 +388,6 @@ mod tests {
             kind: Template::Rebuild,
             state: State::Failed,
             source: Source::Manual,
-            parent_id: None,
             reason: "manual".to_owned(),
             enqueued_at: 0,
             started_at: Some(1),