ci: remove automerge label workflow in favor of GitHub native auto-merge (#544)

## Summary

Removes the custom `automerge.yml` workflow and its `automerge` label
flow. GitHub now provides native auto-merge that merges a PR
automatically once required CI checks pass, making the custom workflow
redundant.

## Changes

- Delete `.github/workflows/automerge.yml`

## Follow-up (manual, outside this PR)

- Enable **Settings → General → Allow auto-merge**.
- Add a branch protection rule requiring the CI check so native
auto-merge waits for it.
- Remove the now-unused `MERGE_TOKEN` secret if nothing else uses it.
- Use **Enable auto-merge** per PR (or `gh pr merge --auto --squash`).

.github/workflows/automerge.yml

@@ -1,57 +0,0 @@
-name: automerge
-
-on:
-  workflow_run:
-    workflows: ['CI']
-    types: [completed]
-
-jobs:
-  automerge:
-    if: >
-      github.event.workflow_run.conclusion == 'success' &&
-      github.event.workflow_run.event == 'pull_request'
-    runs-on: ubuntu-latest
-    permissions:
-      contents: write
-      pull-requests: write
-    steps:
-      - name: Merge PR with automerge label
-        env:
-          GH_TOKEN: ${{ secrets.MERGE_TOKEN }}
-          REPO: ${{ github.repository }}
-        run: |
-          HEAD_BRANCH="${{ github.event.workflow_run.head_branch }}"
-
-          PR_NUMBER=$(gh pr list --repo "$REPO" --head "$HEAD_BRANCH" --state open --json number --jq '.[0].number')
-
-          if [ -z "$PR_NUMBER" ] || [ "$PR_NUMBER" = "null" ]; then
-            echo "No open PR found for branch $HEAD_BRANCH"
-            exit 0
-          fi
-
-          HAS_LABEL=$(gh pr view "$PR_NUMBER" --repo "$REPO" --json labels --jq '[.labels[].name] | map(select(. == "automerge")) | length')
-
-          if [ "$HAS_LABEL" -eq 0 ]; then
-            echo "PR #$PR_NUMBER does not have automerge label, skipping"
-            exit 0
-          fi
-
-          # Ensure the CI run matches the PR's latest commit to avoid merging stale results
-          CI_SHA="${{ github.event.workflow_run.head_sha }}"
-          PR_SHA=$(gh pr view "$PR_NUMBER" --repo "$REPO" --json headRefOid --jq '.headRefOid')
-
-          if [ "$CI_SHA" != "$PR_SHA" ]; then
-            echo "CI ran on $CI_SHA but PR head is $PR_SHA, skipping"
-            exit 0
-          fi
-
-          # Ensure no checks have failed on the PR
-          FAILED=$(gh pr checks "$PR_NUMBER" --repo "$REPO" --json state --jq '[.[] | select(.state == "FAILURE")] | length')
-
-          if [ "$FAILED" -gt 0 ]; then
-            echo "PR #$PR_NUMBER has $FAILED failed check(s), skipping merge"
-            exit 0
-          fi
-
-          echo "PR #$PR_NUMBER has automerge label and all checks passed. Merging..."
-          gh pr merge "$PR_NUMBER" --squash --repo "$REPO" --delete-branch