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

nix/agent-modules/bao.nix

@@ -14,8 +14,7 @@
 # is why: the host file is `0600` to the hive daemon and this unit runs as the
 # unprivileged agent user. nspawn's `--load-credential` is read by the
 # container manager as root and re-exposed under this unit's own `User=`; a
-# bind would deliver a private key this user cannot open. Same answer, same
-# reason, as ./queue.nix's credential pair.
+# bind would deliver a private key this user cannot open.
 #
 # This is the courier's other end: the hive-side collector has no purpose
 # without it, so the two ship together and neither is reachable alone.
@@ -63,12 +62,11 @@ let
   # Where the identity lives in the store, spelled from the same pieces the
   # publisher uses. `swarm_secret_client::mtls::identity_path` builds
   # `swarm/agents/<agent>/bao-mtls` and `path::MOUNT` is `secret`; this
-  # literal is the nix half of that one agreement, exactly as
-  # ../host-modules/glue-queue-agent-credential.nix spells its own.
+  # literal is the nix half of that one agreement.
   identityPath = "secret/swarm/agents/${agentName}/bao-mtls";
 
   # The address is the whole switch — no separate `enable`, the same shape
-  # ./queue.nix and ./logs.nix gate themselves with. A hive that has a store
+  # ./logs.nix gates itself with. A hive that has a store
   # forwards its address and every agent on it gets the check; a hive that has
   # none forwards nothing and no agent does. An `enable` beside it would be a
   # knob whose only correct setting is whatever the address already says, and
@@ -153,7 +151,7 @@ in
         UMask = "0377";
         # Bare ids, no paths: the terse `LoadCredential=` form that inherits a
         # credential the service *manager* received, which is what the
-        # container manager passed in. ./queue.nix states the same shape.
+        # container manager passed in.
         LoadCredential = [
           certCredential
           keyCredential