drop the shared hive agent login

Every agent now connects to the queue and reads logs with its own secret
from the store (#5181), so the per-hive `hive-<h>-agent` OIDC client has
no reader left. Remove it end to end:

- hive-c0re no longer forwards `hive-queue-agent-{secret,client-id}` into
  containers, and no longer forwards HIVE_AGENT_OIDC_TOKEN_ENDPOINT; the
  agent module that loaded them (nix/agent-modules/queue.nix) is gone, its
  options refused with a removal message.
- The host reader (glue-queue-agent-credential.nix, swarm-bao-queue-agent),
  its store role and policy unit (swarm-bao-queue-agent-policy), its PKI
  leaf and the deploy.bao.queueAgent* / queue.agentCredentialDir options
  are gone; the options are refused with a removal message.
- authelia mints no `hive-<h>-agent` client, and swarm-secret-publish no
  longer writes swarm/hives/<h>/queue/agent.
- swarm-secret-client drops queue::agent_client_path / queue::Credential.
- HIVE_AGENT_NATS_URL is now gated on the agents' queue address alone.

The log store's authelia check stays for operators. swarm-nats-auth still
refuses a `-agent` client id; that refusal and `agentClientSuffix` are left
for a follow-up.

Refs #4459
Refs #4765

docs/swarm/README.md

@@ -303,8 +303,7 @@ subjects it publishes to, the credential may answer the one request it was just
 sent, which is how a command gets its reply. The harness and the subagent daemon
 connect with `_INBOX.agent.<agent>` as their inbox prefix, so the replies to the
 icon write and the schedule read arrive. An agent can't read another agent's
-terminal, state, input, commands or replies. The queue refuses the hive's shared
-agent client outright.
+terminal, state, input, commands or replies.
 
 Hives and the swarm controller get a subscribe allow-list too, each with its
 own reply inbox `_INBOX.client.<client-id>.>`, which `swarm_queue_client::connect`