swarm: subagent-status KV bucket, the run state of each subagent

One key per subagent (<agent>.<subagent>) holding running/finished, since,
exit, model, effort and the publish time. The controller creates the bucket,
and the agent credential may publish its own keys. Nothing reads or writes
it yet; the daemon publisher and the controller list follow.

swarm-queue-client/src/subagent_status.rs

@@ -0,0 +1,20 @@
+//! The connection side of the subagent status KV bucket: opening it. The
+//! bucket's name, keys and value are `swarm_wire::subagent_status`.
+
+/// Open the subagent status bucket, creating it if nothing has yet.
+///
+/// `history: 1`: a reader wants each subagent's current state, not every
+/// state it has ever been in.
+#[cfg(feature = "kv")]
+pub async fn open_or_create(
+    client: &async_nats::Client,
+) -> Result<async_nats::jetstream::kv::Store, crate::Error> {
+    let js = async_nats::jetstream::new(client.clone());
+    crate::kv::open_or_create(
+        &js,
+        swarm_wire::subagent_status::BUCKET,
+        "subagent-status",
+        "Whether each subagent is running, as its daemon last published".to_owned(),
+    )
+    .await
+}