fix(ci): grant migration-lock pull-requests read on GITHUB_TOKEN

Checkout fixed the missing .git, but `gh pr list` then failed with
"GraphQL: Resource not accessible by integration (repository.pullRequests)"
— the default token needs an explicit permission grant.

.github/workflows/ci-pr.yml

@@ -87,6 +87,10 @@ jobs:
     # chain; the second PR must wait or rebase.
     runs-on: ubuntu-latest
     timeout-minutes: 5
+    permissions:
+      # gh pr list against the default GITHUB_TOKEN.
+      contents: read
+      pull-requests: read
     steps:
       # gh expands --json files via git, so it needs the repo checked out.
       - uses: actions/checkout@v4