Make agent creation swarm-only and refuse a name placed on another hive

swarm-controller's POST /api/agents now refuses (409) a name the swarm
has already placed on a different hive: a non-Destroyed declaration in
that hive's wanted state, or a SetAgentWanted node still queued for it.
The same name on the same hive is that agent being re-created and goes
through. A wanted state that cannot be read refuses (503/500) instead of
reading as "placed nowhere". Creations are serialised from that read to
the graph insert so two concurrent creations of one name cannot both
pass.

Hive-level creation is removed: hivectl `agent create` / `request-create`,
HostRequest::Spawn / RequestSpawn, the dashboard POST /api/request-spawn
route, and ApprovalKind::Spawn with its approve/resolve arms and the
approval-carrying `templates::spawn`. The swarm path (deploy request or
wanted-state sweep -> queue_first_deploy -> templates::first_deploy) used
none of them. Old `spawn` approval rows are skipped by collect_lenient,
as `init_config` rows were in a3b672d1.

policy.rs's comment on agent_object_name stated swarm-wide name
uniqueness as a fact; it now says where it is enforced and what that
check cannot see.

Refs #4396

hive-c0re/src/job_queue/model.rs

@@ -122,7 +122,7 @@ pub enum NodeKind {
     /// node-inventory row for its three responsibilities and why it isn't
     /// named `AbortDeploy`.
     DeployTail { agent: String, approval_id: i64 },
-    /// Tail node of an approval-carrying DAG (spawn / opaque deploy /
+    /// Tail node of an approval-carrying DAG (opaque deploy /
     /// config-PR merge): resolve the approval row from how the work ended.
     ResolveApproval {
         approval_id: i64,