swarm: carry the agent queue credential from the host into the harness

hive-c0re stats the two files `swarm-bao-queue-agent` lands and forwards
them into every agent container as systemd credentials, and the harness
resolves a `QueueConfig` out of them at boot. Nothing connects yet.

A credential and not a bind mount, and the mode is what forces it: the
secret is root:0600 and the harness runs as the unprivileged agent user,
so a bind would deliver a file that user cannot open. nspawn's
`--load-credential` is read by the container manager as root and
re-exposed under the consuming unit's own `User=`. hive-c0re never reads
the bytes either way, which is just as well — it runs as `hive-core`.

Absent files stay legal and become visible rather than silent: the
publisher lives on the authelia host and mints on its first boot, so
"nothing at that path" is the ordinary early state of a swarm. c0re
forwards nothing and logs why; the harness logs that it has no queue.

The client id comes out of the delivered file rather than being rebuilt
from `hiveName` in nix, which is the agreement `swarm-secret-client`
states. `QueueConfig::from_env` wants it as a value, so the harness reads
the file itself — assigning the variable instead would need
`std::env::set_var` in a process that has already spawned threads.

Refs #3805

hive-agent/Cargo.toml

@@ -31,6 +31,9 @@ rusqlite.workspace = true
 schemars.workspace = true
 serde.workspace = true
 serde_json.workspace = true
+# Bare: `kv`/`notices` name buckets and streams this harness opens neither
+# end of. All it wants from the crate is `QueueConfig` and, next, a connect.
+swarm-queue-client.workspace = true
 tokio.workspace = true
 tokio-stream.workspace = true
 tower-http.workspace = true