ci docs: the shared daemon is per runner (runner-a1 runs several jobs at once)

Task logs of run 740: runner-a1 received task 2721 (smoke-studio) at
22:53:02Z and task 2722 (build-variant) at 22:58:38Z while 2721 was still
running. Other runners (runner-1/2/3) have capacity 1. 12e66aa said all
jobs share one daemon; corrected in CHANGELOG, AGENTS.md, the script
header and the lint.yml comment. No behaviour change.

CHANGELOG.md

@@ -17,8 +17,9 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
 
 - **CI: a smoke job's image could be deleted mid-test by another job's disk
   reclaim.** This is why v1.12.2's `-studio` tags were not published by its
-  tag run. All `docker-publish.yml` jobs share the runner's Docker daemon, and
-  each began with `docker system prune -af --volumes`, which removes every
+  tag run. Jobs that land on the same runner share its Docker daemon (in run
+  740, runner-a1 ran `smoke-studio` and `build-variant` at the same time), and
+  each job began with `docker system prune -af --volumes`, which removes every
   image that no container is using at that moment. A smoke image is unused
   between `smoke-test.sh`'s one-shot `docker run`s. In run 740, `smoke`
   passed and `build-variant` started. Its prune logged