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/job_queue/templates.rs

@@ -19,7 +19,7 @@
 //! rebuild(a):        Prebuild(a) → StopForUpdate(a) → Swap(a) →(any) Reconcile(a)
 //! spawn(a):          Provision(a) → Create(a) → WriteDropin(a) → Reconcile(a)  [wanted=Up at approve]
 //! perm-change(a):    WritePermFile(a) → «rebuild subgraph»
-//! meta-update(inp):  MetaLock(inp) → «fan-out rebuild(a) per affected a»
+//! meta-update(inp):  MetaLock(inp) →«in-DAG rebuild subgraph per affected a»
 //! ```
 //!
 //! For the dynamic power-op shapes (`stop` / `start` / `restart`, built from
@@ -83,18 +83,11 @@ pub(crate) fn rebuild_nodes(agent: &str, relock: bool, base: u32) -> Vec<NodeSpe
 /// when `wanted = Offline` (a rebuild of a deliberately-stopped agent
 /// leaves it stopped). `relock = false` only for meta-update cascade
 /// children.
-pub fn rebuild(
-    agent: &str,
-    source: Source,
-    reason: String,
-    parent_id: Option<u64>,
-    relock: bool,
-) -> DagSpec {
+pub fn rebuild(agent: &str, source: Source, reason: String, relock: bool) -> DagSpec {
     DagSpec {
         template: Template::Rebuild,
         source,
         reason,
-        parent_id,
         approval_id: None,
         inputs: Vec::new(),
         perm_payload: None,
@@ -111,7 +104,6 @@ pub fn approval_deploy(agent: &str, approval_id: i64, reason: String) -> DagSpec
         template: Template::Rebuild,
         source: Source::Approval,
         reason,
-        parent_id: None,
         approval_id: Some(approval_id),
         inputs: Vec::new(),
         perm_payload: None,
@@ -135,7 +127,6 @@ pub fn reconcile_only(
         template,
         source,
         reason,
-        parent_id: None,
         approval_id: None,
         inputs: Vec::new(),
         perm_payload: None,
@@ -153,7 +144,6 @@ pub fn spawn(agent: &str, approval_id: i64, reason: String) -> DagSpec {
         template: Template::Spawn,
         source: Source::Approval,
         reason,
-        parent_id: None,
         approval_id: Some(approval_id),
         inputs: Vec::new(),
         perm_payload: None,
@@ -177,7 +167,6 @@ pub fn perm_change(agent: &str, source: Source, reason: String, payload: PermPay
         template: Template::PermChange,
         source,
         reason,
-        parent_id: None,
         approval_id: None,
         inputs: Vec::new(),
         perm_payload: Some(payload),
@@ -190,7 +179,7 @@ pub fn perm_change(agent: &str, source: Source, reason: String, payload: PermPay
 /// per affected agent into *this same* DAG on completion (via
 /// `append_subgraph`) — appended *after* the bump lands so their prebuilds
 /// run against the post-bump lock, and a failed bump appends nothing
-/// (replacing the old fan-out-child-DAGs + `cancel_children` dance).
+/// (replacing the old fan-out-child-DAGs dance).
 /// `transient = Rebuilding` because those appended subgraphs are rebuilds:
 /// it's applied per-agent at claim time (the `MetaLock` head needs no lease,
 /// so the "hyperhive" pseudo-agent gets no pill), giving each cascade agent
@@ -206,7 +195,6 @@ pub fn meta_update(
         template: Template::MetaUpdate,
         source,
         reason,
-        parent_id: None,
         approval_id,
         inputs,
         perm_payload: None,