ci: run on merge groups

[octalide]

ci.yml also runs on merge_group, so a merge queue can require the gate check on the queued merge commit.

Decisions

• The trigger takes no types, so it runs on checks_requested, the only merge group event.
• The concurrency group needs no change. A merge group runs on its own gh-readonly-queue/... ref, and cancel-in-progress stays limited to pull requests, so a queued run is never cancelled by a later one.
• In the packages, the header comment now names merge groups too, so it stays accurate.

Ran

actionlint on ci.yml. CI on this pull request passed. The merge group path runs once the queue ruleset is enabled.