Summary

docs.pytest.org/en/stable is built from the current maintenance branch (e.g. 9.0.x), not main. The weekly plugin list update only creates PRs against main, so the stable docs always show a frozen plugin list from the time of the release cut.

Fix

Add a second job update-plugin-list-stable to the workflow that:

1. Dynamically detects the latest maintenance branch (highest N.N.x branch) via git ls-remote
2. Checks it out, runs the same tox -e update-plugin-list update
3. Creates a draft PR targeting that maintenance branch

The branch detection is version-sorted so it correctly finds e.g. 9.0.x over 8.4.x.

Also adds the missing if: steps.pr.outputs.pull-request-number guard to the "Instruct maintainers" step in the original job (without it the gh pr comment call would fail when there are no plugin list changes).

Testing

The logic can be verified locally:

git ls-remote --heads https://github.com/pytest-dev/pytest.git \
  | grep -oE 'refs/heads/[0-9]+\.[0-9]+\.x' \
  | sed 's|refs/heads/||' \
  | sort -t. -k1,1n -k2,2n \
  | tail -1
# → 9.0.x