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-host-sock/src/lib.rs

@@ -78,9 +78,9 @@ pub enum HostRequest {
     /// matrix GUI disabled). Backs `hivectl open` + the federation
     /// peer-config block (which reads the bare `domain`).
     Urls,
-    /// Fetch one job-queue DAG (plus its live fan-out children, linked
-    /// via `parent_id`) by id — the polling surface behind `hivectl`'s
-    /// wait/progress loop. Result: [`HostResponse::dags`].
+    /// Fetch one job-queue DAG by id — the polling surface behind
+    /// `hivectl`'s wait/progress loop. A multi-step op is a single DAG
+    /// (its whole graph in `nodes`). Result: [`HostResponse::dags`].
     QueueDag { id: u64 },
     /// List pending approval requests.
     Pending,