refactor(#3255): a constant, not a module with an essay

Review call: 46 lines of documentation around a single constant, part of
it already stale. The worst paragraph explained why the earlier per-hive
shape had been justified wrongly — history of a design that never
shipped, written into the file within an hour of that design being
dropped. A file is not a changelog; why it was wrong belongs in the PR.

The constant moves to lib.rs beside the status bucket name, keeping only
the rationale that stays true: three crates must agree on the string, and
the one that agrees hardest speaks neither jetstream nor kv, which is why
it cannot sit behind a feature gate.

status earns a module of its own because it holds a bucket name AND the
functions that open it. This held a constant.

hive-c0re/src/swarm_status.rs

@@ -212,7 +212,7 @@ async fn drain_swarm_events(
     // One subject for the whole swarm, so this hive's own name never enters
     // it: the controller publishes once and core NATS fans out to whoever is
     // subscribed.
-    let subject = swarm_queue_client::knowledge::SUBJECT;
+    let subject = swarm_queue_client::KNOWLEDGE_SUBJECT;
     let mut sub = match client.subscribe(subject).await {
         Ok(sub) => sub,
         Err(e) => {