Add automerge GitHub Action (#5)

PRs labeled 'automerge' will squash-merge when all checks pass.
Uses pascalgn/automerge-action. No branch protection needed.

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

.github/workflows/automerge.yml

@@ -0,0 +1,29 @@
+name: Automerge
+
+on:
+  check_suite:
+    types: [completed]
+  pull_request:
+    types: [labeled]
+
+jobs:
+  automerge:
+    name: Automerge PR
+    runs-on: ubuntu-latest
+    if: >
+      github.event_name == 'pull_request' ||
+      (github.event_name == 'check_suite' && github.event.check_suite.conclusion == 'success')
+    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: "squash"
+          MERGE_DELETE_BRANCH: "true"
+          UPDATE_METHOD: "rebase"