Replace automerge action with GitHub native merge queue

The automerge GitHub Action (pascalgn/automerge-action) is removed in
favor of GitHub's built-in merge queue, which is now enabled on the
main branch ruleset.

- Delete automerge.yml workflow
- Add merge_group trigger to CI so it runs on queue candidates
- Remove workflow_run trigger for Automerge from CD
- Update CLAUDE.md to document gh pr merge --merge-queue

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -5,6 +5,7 @@ on:
     branches: [main]
   pull_request:
     branches: [main]
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}