swarmctl: drop agent mint-identity and its controller route

Re-creating an agent mints its store identity the same way, so the
backfill verb and the `POST /api/agents/{name}/identity` route that only
it called go. The `MintAgentIdentity` job node stays: agent creation and
the renewal pass still queue it.

Part of #4765 (c).

docs/getting-started/setup.md

@@ -95,7 +95,7 @@ hive-c0re creates the manager agent, ruth, on its own at startup — so
 unlike agents made through the swarm, she has no store identity yet:
 
 ```bash
-swarmctl agent mint-identity ruth   # on the swarm-controller's host
+swarmctl agent create ruth --hive <hive>   # on the swarm-controller's host
 hivectl agent ruth rebuild          # on ruth's hive: hands her the identity
 ```