The swarm always has exactly one queue; a hive can only lack its
address. Reworded every prose site this PR added that stated or
implied the opposite, to name what is actually absent (coordinates,
credential, or address) instead of the queue itself.
Refs #3805
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
