feat(ci): trigger gh-gcs release automatically from cd.yaml

[shouze]

Context

Part of the gh-gcs epic (#93).

Once the fulll/gh-gcs release workflow exists (#96), cd.yaml must dispatch a repository_dispatch event to fulll/gh-gcs at the end of a successful release so that the extension is always kept in sync without any manual step.

Goal

Add a final step to the release job in .github/workflows/cd.yaml:

- name: Trigger gh-gcs release
  env:
    GH_TOKEN: ${{ secrets.GH_GCS_DISPATCH_TOKEN }}
  run: |
    gh api repos/fulll/gh-gcs/dispatches \
      --method POST \
      --field event_type=new-release \
      --field 'client_payload[version]=${{ github.ref_name }}'

Required secret

GH_GCS_DISPATCH_TOKEN: a GitHub PAT (or fine-grained token) scoped to fulll/gh-gcs with contents: write permission. Must be added to the fulll/github-code-search repository secrets.

The step must run after the release assets are fully uploaded and the GitHub Release is created.

Acceptance criteria

• Pushing a vX.Y.Z tag to github-code-search automatically triggers the gh-gcs release workflow
• The gh-gcs release is created with the matching version tag within ~5 min of the github-code-search release
• A missing or invalid GH_GCS_DISPATCH_TOKEN fails visibly in the workflow log (non-silent failure)
• Manual workflow_dispatch on gh-gcs still works as a fallback