ci: report ci-shared-status-check on merge_group (#23276)

Follow-up to #23275. `ci-shared` was the one required check left off
that batch, so `ci-shared-status-check` still sits at "Expected -
Waiting for status to be reported" and blocks the merge queue.

Same fix as the other workflows: add a `merge_group` trigger and gate
`changed-files-check` with `if: github.event_name != 'merge_group'`. On
a queued candidate, `changed-files-check` skips, `shared-test` (gated on
`any_changed`) cascades to skipped, and the `always()`-gated
`ci-shared-status-check` job reports success in seconds. The full suite
still runs on `pull_request`.

---
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.github/workflows/ci-shared.yaml

@@ -2,6 +2,7 @@ name: CI Shared
 
 on:
   pull_request:
+  merge_group:
 
 permissions:
   contents: read
@@ -12,6 +13,7 @@ concurrency:
 
 jobs:
   changed-files-check:
+    if: github.event_name != 'merge_group'
     uses: ./.github/workflows/changed-files.yaml
     with:
       files: |