ci: add stlc promote workflow

.github/workflows/stlc-promote.yml

@@ -0,0 +1,37 @@
+# Promote stage — install this file at .github/workflows/stlc-promote.yml in
+# EACH SDK repo (linq-go, linq-node, linq-python). It is repo-agnostic.
+#
+# linq-sdks' "Build SDKs" workflow force-pushes regenerated code to the
+# `stainless/release` branch here. This workflow turns that branch into a
+# release PR against main. The force-push updates an already-open PR in place,
+# so this only needs to create the PR the first time.
+#
+# Auth: the org-level STLC_ACTIONS_PAT secret (same PAT linq-sdks uses). A PAT,
+# not GITHUB_TOKEN, so the release PR triggers this repo's CI checks.
+
+name: Open release PR
+
+on:
+  push:
+    branches: [stainless/release]
+
+jobs:
+  promote:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Open or update the release PR
+        env:
+          GH_TOKEN: ${{ secrets.STLC_ACTIONS_PAT }}
+          REPO: ${{ github.repository }}
+        run: |
+          EXISTING=$(gh pr list --repo "$REPO" \
+            --head stainless/release --base main --state open \
+            --json number --jq '.[0].number // empty')
+          if [ -n "$EXISTING" ]; then
+            echo "Release PR #${EXISTING} already open — the force-push updated it."
+            exit 0
+          fi
+          gh pr create --repo "$REPO" \
+            --base main --head stainless/release \
+            --title "Release: regenerated SDK" \
+            --body "Regenerated by stlc from an updated API spec. Review the diff and merge to cut a release (release-please versions and publishes on merge)."