swarm: wire the agents' queue coordinates and credential through the modules

The host end: `HIVE_C0RE_AGENT_QUEUE_CREDENTIAL_DIR` tells the daemon
where the reader unit put the files, and a new
`deploy.hive-controller.queue.agentNatsUrl` says where the queue is as an
agent *container* reaches it. That address defaults to the bridge one and
never to loopback — `statusPublish.natsUrl` beside it is loopback and
correct, because hive-c0re shares the host netns and an agent does not.
Paired with the swarm's token endpoint, gated together, and forwarded by
`hive_c0re::meta` as both an env var and an agent option: the harness
reads the variable at runtime, its unit is built from the option.

The agent end: `nix/agent-modules/queue.nix` declares that option pair
and, when set, has the harness unit inherit the two credentials by name.
Bare-id `LoadCredential=` is the terse form documented for inheriting
what the service manager received, and is non-fatal when the credential
is absent — which a hive whose publisher has not run yet needs.

No `HIVE_AGENT_OIDC_CA_FILE`: the meta flake already embeds the hive CA
and the swarm root into each container's trust store at build time, and
reqwest's rustls backend verifies against it.

Refs #3805

nix/agent-modules/default.nix

@@ -33,6 +33,7 @@
     ./network.nix
     ./otel.nix
     ./packages.nix
+    ./queue.nix
     ./user.nix
     ./screen.nix
     ./weston-vnc.nix