ci: bound CI storage; don't bridge image-build jobs

- deploy/runner/prune.sh + windygit-ci-prune.timer (6h): age-based prune
  of the CI-only dind (containers, finished-job volumes, images/builder
  cache >7d) plus a hard 60 GB cap. Only that daemon, over its own TCP
  socket; never the host's Docker. It was 38 GB and unbounded — the same
  class of growth that filled Kit 0 on 09-01.
- pr_status_bridge: jobs named *docker* are not posted. Job containers
  have no daemon by design (I-5), so they are red on every commit; a
  permanent red X teaches everyone to ignore red.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/runner/windygit-ci-prune.service

@@ -0,0 +1,6 @@
+[Unit]
+Description=Windy Git - prune CI-only dind storage (bounded, never the host daemon)
+
+[Service]
+Type=oneshot
+ExecStart=/srv/windygit/src/deploy/runner/prune.sh