ci: arm auto-merge on Fern regeneration PRs

.fernignore

@@ -1 +1,9 @@
 # Specify files that shouldn't be modified by Fern
+#
+# auto-merge.yml is hand-written and arms Fern's own regeneration PRs. On pull_request_target
+# it enables auto-merge (squash) for a pull request opened by fern-api[bot] from an in-repo
+# fern-bot/* branch against the default branch, so the pull request merges itself once main's
+# required status checks pass. The trigger is pull_request_target rather than pull_request so
+# the workflow that arms a pull request is always the copy already on the base branch, never
+# the copy that pull request carries.
+.github/workflows/auto-merge.yml

.github/workflows/auto-merge.yml

@@ -0,0 +1,29 @@
+name: Auto-merge
+
+on:
+  pull_request_target:
+    types: [opened, reopened, synchronize]
+
+permissions:
+  contents: write
+  pull-requests: write
+
+concurrency:
+  group: auto-merge-${{ github.event.pull_request.number }}
+  cancel-in-progress: false
+
+jobs:
+  arm:
+    if: >-
+      github.event.pull_request.user.id == 115122769 &&
+      github.event.pull_request.user.login == 'fern-api[bot]' &&
+      github.event.pull_request.head.repo.full_name == github.repository &&
+      github.event.pull_request.base.ref == github.event.repository.default_branch &&
+      startsWith(github.event.pull_request.head.ref, 'fern-bot/')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Enable auto-merge
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+        run: gh pr merge "$PR_NUMBER" --repo "$GITHUB_REPOSITORY" --auto --squash