ci: subscribe CI to merge_group events (#582)

**Motivation**

GitHub's merge queue runs required checks against a temporary
`gh-readonly-queue/*` branch, and it only reports a workflow as passing
if that workflow subscribes to the `merge_group` event
([docs](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#triggering-merge-group-checks-with-github-actions)).

`Lint` and `Test` live in `CI`, which only listened to `push`,
`pull_request` and `workflow_dispatch`. If those are set as required
checks, the queue would sit waiting on checks that never start.

**Description**

- Add the `merge_group:` trigger to `.github/workflows/ci.yml`.
- Note in the `concurrency` comment that `merge_group` runs carry no
`head_ref`, so the group falls back to the unique `run_id` and queue
runs are never cancelled by a sibling run.

Only `CI` is changed; the other workflows are not required checks for
merging.

Author: MegaRedHand

.github/workflows/ci.yml

@@ -5,6 +5,7 @@ on:
     branches: [main]
   pull_request:
     branches: ["**"]
+  merge_group:
   workflow_dispatch:
 
 # Cancel in-progress runs when a new commit is pushed to the same PR or branch