## Summary
- Fixes merge queue PRs blocking each other by changing the concurrency
group in `ci-merge-queue.yaml`
- The old concurrency group used `merge_group.base_ref` which resolves
to `refs/heads/main` for every PR, causing all merge queue entries to
serialize behind a single concurrency slot
- Now uses `github.ref` (unique per entry:
`refs/heads/gh-readonly-queue/main/pr-NUMBER-SHA`), matching what all
other CI workflows already do
## Recommended ruleset changes (in GitHub Settings > Rules > Rulesets >
"CI Status Checks")
- **Grouping strategy**: Switch `ALLGREEN` to `NONE` -- each PR is still
tested against the correct base (including all PRs ahead of it in the
queue), but failures only affect the failing PR instead of ejecting the
entire group. `max_entries_to_build: 5` still allows parallel
speculative testing.
- **`min_entries_to_merge_wait_minutes`**: Reduce from 5 to 1 -- the
5-minute wait adds unnecessary latency to every merge.
## Test plan
- [ ] Enqueue 2+ PRs in the merge queue and verify both trigger e2e
tests in parallel instead of one blocking the other
