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/dashboard_events.rs

@@ -208,8 +208,8 @@ pub enum DashboardEvent {
     /// snapshot-shape rationale as `TombstonesChanged` /
     /// `MetaInputsChanged`: the list is small, snapshot semantics avoid
     /// the add/remove races a per-row event would have, and the
-    /// dashboard's grouping (`parent_id`) is most naturally re-derived
-    /// from the full list.
+    /// dashboard renders each DAG's multi-agent shape from its `nodes`
+    /// (grouped by `NodeView::agent`) — no cross-DAG grouping needed.
     RebuildQueueChanged { seq: u64, queue: Vec<DagView> },
     /// Full snapshot of all scheduled prompts. Emitted after every
     /// operator mutation (new / edit / cancel / fire-now) and after the