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/persistence.md

@@ -10,8 +10,8 @@ keeps its state, purging it doesn't.**
 - **`DESTR0Y`** (the default action) stops and removes the container
   but keeps everything on disk — config history, claude login, `/state/`
   notes, harness data. The agent shows up as a tombstone (K3PT ST4T3 on
-  the C0R3 page) with a `⊕ R3V1V3` button that recreates it from the
-  kept state, **no re-login needed**.
+  the C0R3 page); `swarmctl agent create` with the same name and hive
+  recreates it from the kept state, **no re-login needed**.
 - **`PURG3`** (opt-in, from the dashboard or `hivectl agent <name>
   destroy --purge`) is `DESTR0Y` plus wiping all of it — config
   history, claude credentials, `/state/` notes, everything. **No
@@ -432,8 +432,8 @@ See [For operators](#for-operators) above for what each action does to
 an agent's state. The mechanics, for completeness:
 
 - `DESTR0Y` also drops the systemd drop-in and fails any pending
-  approvals; the tombstone's `⊕ R3V1V3` button queues a Spawn approval
-  that reuses the kept state on approve.
+  approvals; reviving the agent is `swarmctl agent create` with the same
+  name and hive, which reuses the kept state.
 - `PURG3` wipes `/var/lib/hyperhive/{agents,applied}/<name>/` — the
   union of everything `DESTR0Y` left behind.