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/workers/auto_update.rs

@@ -112,7 +112,6 @@ pub async fn ensure_root_agent(coord: &Arc<Coordinator>) -> Result<()> {
                 MANAGER_NAME,
                 crate::job_queue::Source::AutoUpdate,
                 "manager migration: no applied flake".to_owned(),
-                None,
                 true,
             )) {
                 tracing::warn!(error = ?e, "manager migration rebuild submit failed");
@@ -347,7 +346,6 @@ fn submit_boot_tree(
         template: Template::Boot,
         source: Source::AutoUpdate,
         reason,
-        parent_id: None,
         approval_id: None,
         inputs: Vec::new(),
         perm_payload: None,