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

docs/agent-lifecycle/agent-hierarchy.md

@@ -100,7 +100,8 @@ other agents don't:
 - **Naming/bootstrap** — the manager's broker recipient name, state-dir
   key, and nixos-container name are all `ruth` (container `h-ruth`).
   `hive-c0re` spawns it directly at boot if missing, with no operator
-  approval step — every other agent goes through a `Spawn` approval.
+  approval step — every other agent is created at swarm level
+  (`swarmctl agent create`).
   Roster-wise, `ruth` is just another entry.
 - **Wire-protocol** — the privileged `Request` variants
   (`Kill` / `Start` / `Restart` / `Update`; `GetLogs`) — marked