Retry release-bump push with --rebase on concurrent contention

[niksedk]

Summary

Follow-up to merged #275. When several SE5 plugin workflows are dispatched at the same time (as just happened during the 1.1.0 release run), they all check out the same SHA, all create their bump commits, and only one wins the push. The other 4 failed with ! [rejected] main -> main (fetch first) and had to be retriggered serially.

This PR wraps the bump push in a small retry-with-rebase loop in all 5 prepare jobs:

attempts=0
while ! git push 2>/tmp/push.err; do
  attempts=$((attempts + 1))
  if [ "$attempts" -ge 5 ]; then
    echo "Failed to push after $attempts attempts:" >&2
    cat /tmp/push.err >&2
    exit 1
  fi
  echo "Push rejected (attempt $attempts), pulling --rebase and retrying..."
  git pull --rebase --no-edit
done

Different plugins touch different plugin.json files, so the rebase never conflicts. Same-plugin double-dispatch is not a realistic case (you wouldn't release the same plugin twice in parallel).

Test plan

• After merge, dispatch all 5 release workflows in quick succession again — all 5 prepare jobs should now succeed, with the later ones logging "Push rejected (attempt N), pulling --rebase and retrying...".
• Single-workflow release path is unchanged (one push attempt, no retry).

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