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/coordinator.rs

@@ -841,7 +841,7 @@ impl Coordinator {
     /// Call after any mutation that could affect what
     /// `nixos-container list` returns or what a row's
     /// `running` / `needs_update` / `needs_login` / `deployed_sha`
-    /// resolves to — lifecycle ops, destroy, approve (post-spawn),
+    /// resolves to — lifecycle ops, destroy,
     /// rebuild, meta-update, and the crash-watcher's periodic poll.
     /// Cheap when nothing changed (one `nixos-container list` + a
     /// `HashMap` diff + zero emits).