ci: checkout repo in the bundle job so release notes can read CHANGELOG

The Combine-artifacts job only downloaded build artifacts — it never checked
out the source, so the release-notes step's `awk … CHANGELOG.md` failed with
exit 2 (no such file). Add actions/checkout at the tagged ref.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: DaveBugg

.github/workflows/release.yml

@@ -213,6 +213,9 @@ jobs:
     needs: [frontend, backend, naive]
     runs-on: ubuntu-latest
     steps:
+      # Needed so the release-notes step can read CHANGELOG.md at this tag.
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+
       - uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
         with:
           # Restrict to our own artifacts. `build-push-action` also