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/cd.yml

@@ -1,16 +1,11 @@
 name: CD
 
 # Triggers:
-# - push: normal merges (manual gh pr merge)
-# - workflow_run: after automerge completes (GITHUB_TOKEN merges don't trigger push)
+# - push: merges via merge queue or manual gh pr merge
 # - workflow_dispatch: manual trigger via gh workflow run cd.yml
 on:
   push:
     branches: [main]
-  workflow_run:
-    workflows: ["Automerge"]
-    branches: [main]
-    types: [completed]
   workflow_dispatch: {}
 
 concurrency:
@@ -21,7 +16,6 @@ jobs:
   build-images:
     name: Build & Push Docker Images
     runs-on: ubuntu-latest
-    if: github.event_name != 'workflow_run' || github.event.workflow_run.conclusion == 'success'
     permissions:
       contents: read
       packages: write