docs(run-engine): cut the ck fair scheduling release note down to the change

The note ran to three sentences, advertised a server env var a user cannot set,
and described the pass-1 window degradation. .server-changes/README.md asks for a
one-line description of behaviour rather than implementation, and says that
needing a paragraph usually means you are describing the implementation.

Reported by Devin on #4367; this is its suggested wording.

Author: 1stvamp

.server-changes/2026-07-24-ck-fair-scheduling.md

@@ -3,4 +3,4 @@ area: webapp
 type: feature
 ---
 
-The run queue can now serve concurrency keys fairly, so one key with a large backlog no longer starves runs waiting on other keys. It is opt-in and off by default. While a lot of keys on the same queue are all waiting on retry backoffs or future start times, ordering falls back to arrival order until they clear, so runs keep flowing but the fairness guarantee is relaxed for that period.
+One concurrency key with a large backlog no longer holds up runs waiting on other keys on the same queue.