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-sh4re/src/jobs.rs

@@ -189,8 +189,6 @@ pub struct DagView {
     /// `queued` / `cancelled` / `done`.
     pub state: State,
     pub source: Source,
-    #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub parent_id: Option<u64>,
     pub reason: String,
     pub enqueued_at: i64,
     #[serde(default, skip_serializing_if = "Option::is_none")]