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

swarmctl/src/main.rs

@@ -206,7 +206,9 @@ struct AgentCreateArgs {
     /// Name for the new agent: 1–63 characters of `[a-z0-9-]`.
     ///
     /// Becomes an SSO subject, a forge user and a repository name, so
-    /// it's validated here before queuing.
+    /// it's validated here before queuing. The controller refuses a name
+    /// the swarm has already placed on a different hive; the same name on
+    /// the same hive re-creates that agent.
     name: String,
     /// Hive in this swarm to deploy the agent to.
     ///