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.

swarm-controller/src/webhook.rs

@@ -416,7 +416,7 @@ async fn announce_knowledge_change(state: &AppState) {
         return;
     };
     let client = status.queue_client();
-    let subject = swarm_queue_client::knowledge::SUBJECT;
+    let subject = swarm_queue_client::KNOWLEDGE_SUBJECT;
 
     // One publish, not one per hive: every subscriber gets the same empty
     // event, so the roster is not consulted at all. The controller does not