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/automerge.yml

@@ -1,51 +0,0 @@
-name: Automerge
-
-# Triggers:
-# - pull_request: fires when PR is opened, labeled, updated, etc.
-#   Handles the case where the label is added after CI already passed.
-# - workflow_run (CI): fires after the CI workflow completes.
-#   This is needed because check_suite and status events don't reliably
-#   fire for workflows on the same commit (GitHub prevents infinite loops).
-#   Without this, the automerge action never retries after CI passes.
-on:
-  pull_request:
-    types:
-      - labeled
-      - unlabeled
-      - synchronize
-      - opened
-      - edited
-      - ready_for_review
-      - reopened
-      - unlocked
-  pull_request_review:
-    types:
-      - submitted
-  workflow_run:
-    workflows: ["CI"]
-    types:
-      - completed
-
-env:
-  FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
-
-jobs:
-  automerge:
-    name: Automerge PR
-    runs-on: ubuntu-latest
-    permissions:
-      contents: write
-      pull-requests: write
-    steps:
-      - uses: actions/checkout@v6
-
-      - name: Automerge
-        uses: pascalgn/automerge-action@v0.16.4
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          MERGE_LABELS: "automerge"
-          MERGE_METHOD: "merge"
-          MERGE_DELETE_BRANCH: "true"
-          MERGE_READY_STATE: "clean"
-          UPDATE_METHOD: "rebase"
-          BASE_BRANCHES: "main"

.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

.github/workflows/ci.yml

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

CLAUDE.md

@@ -110,11 +110,11 @@ pnpm db:studio        # Open Drizzle Studio (DB browser)
 ### Opening a PR
 - Create a feature branch: `git checkout -b <descriptive-name>`
 - Keep PRs focused — one logical change per PR
-- Add the `automerge` label to auto-merge when CI passes:
+- Use the merge queue to auto-merge when CI passes:
   ```bash
-  gh pr create --title "..." --body "..." --label automerge
+  gh pr create --title "..." --body "..."
+  gh pr merge --merge-queue
   ```
-- PRs with `automerge` label merge and delete the branch automatically
 
 ### Stacking PRs (for fast local iteration)
 When working on sequential tasks, stack branches locally:
@@ -126,8 +126,8 @@ main → feature-a → feature-b → feature-c
 - When feature-a merges, rebase feature-b onto main
 - This keeps you unblocked while PRs are in CI
 
-### Important: Do not push to a branch after its PR auto-merges
-With the `automerge` label, PRs squash-merge as soon as CI passes. If you
+### Important: Do not push to a branch after its PR merges via merge queue
+Once a PR enters the merge queue it will merge as soon as CI passes. If you
 push additional commits to the branch after that, those commits are orphaned
 — they won't be on main. Always check `gh pr view <number> --json state`
 before pushing to an existing PR branch.