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.

scripts/ci-reclaim-docker.sh

@@ -9,8 +9,10 @@
 #   bash scripts/ci-reclaim-docker.sh assert-protected IMG  # smoke preflight
 #   bash scripts/ci-reclaim-docker.sh lint WORKFLOW...      # lint.yml guard
 #
-# WHY THIS EXISTS. The jobs share ONE Docker daemon (the runner's socket), and
-# each used to start with `docker system prune -af --volumes`. That removes
+# WHY THIS EXISTS. Jobs that land on the same runner share its Docker daemon
+# (runner-a1 runs several at once: in run 740 it had smoke-studio and
+# build-variant going together), and each job used to start with
+# `docker system prune -af --volumes`. That removes
 # every image no container is using at that instant, and a smoke image is
 # exactly that for most of its life: smoke-test.sh runs one `docker run --rm`
 # per assertion, so between assertions nothing holds the image. v1.12.2 run 740