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-sh4re/src/approvals.rs

@@ -46,9 +46,6 @@ pub struct Approval {
 #[serde(rename_all = "snake_case")]
 #[strum(serialize_all = "snake_case")]
 pub enum ApprovalKind {
-    /// Create + start a new sub-agent container with the given name
-    /// (under the default `agent.nix` template).
-    Spawn,
     /// Run `nix flake update [inputs...]` on the meta flake and commit
     /// the resulting lock changes.
     UpdateMetaInputs,