ci: run playbook guard on every PR so it can be required (#611)

The check-branches guard had a paths filter, so it only ran when
local-antora-playbook.yml changed. That meant it couldn't be made a
required status check: a required check skipped by a paths filter leaves
unrelated PRs stuck pending.

Remove the paths filters so the guard runs on every PR (and on push to
main). It passes fast when the playbook is standard. This makes it safe
to mark check-branches as a required status check, which would have
blocked PR #608 from merging with a non-standard branch reference.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Joyce Fee <102751339+Feediver1@users.noreply.github.com>

Author: 3 people

.github/workflows/check-playbook.yml

@@ -1,13 +1,12 @@
 ---
 name: Check playbook branches
+# Runs on every PR (no paths filter) so it can be a required status check:
+# a required check that is skipped by a paths filter leaves PRs stuck pending.
+# It passes fast when the playbook is unchanged or standard.
 on:
   pull_request:
-    paths:
-      - local-antora-playbook.yml
   push:
     branches: [main]
-    paths:
-      - local-antora-playbook.yml
 
 jobs:
   check-branches: