ci(stlc): promote as soon as an stlc build lands on main

A codegen commit sat on staging main between daily promotes while a
production PR (#844) squash-merged on the old base, forking the trunks.
Back-sync and promote both refuse a forked pair, so back-sync failed on
every poll until the fork was healed by hand with a merge commit.

Trigger the promote on push to main, gated to commits carrying the
Stainless-Generated-From trailer, which shrinks that window from up to
a day to seconds. The daily schedule stays as a backstop.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: tode-rl

.github/workflows/stlc-promote.yml

@@ -3,11 +3,14 @@ name: Promote SDKs
 # Promote staging to production by fast-forwarding production main up to staging,
 # preserving SHAs so the trunks stay identical and linear (nothing to heal on the
 # next stlc build). This is a git-only reconciliation, not a release: release-please
-# owns the actual version bump/publish gate on production. Runs daily so unpromoted
-# staging content can't sit long enough to fork against an independent production
-# push; workflow_dispatch remains for an eager promote. The fast-forward-only guard
-# below makes every run a safe no-op when there's nothing to promote.
+# owns the actual version bump/publish gate on production. Runs as soon as an stlc
+# build lands on staging main, because any production merge before the promote forks
+# the trunks and blocks both directions; the daily schedule is a backstop and
+# workflow_dispatch an eager promote. The fast-forward-only guard below makes every
+# run a safe no-op when there's nothing to promote.
 on:
+  push:
+    branches: [main]
   schedule:
     # 8am PT (15:00 UTC); shifts to 7am during PST since GitHub cron doesn't track DST.
     - cron: '0 15 * * *'
@@ -20,7 +23,11 @@ jobs:
   promote:
     # Runner comes from the STLC_RUNNER repo/org variable when set; defaults to GitHub-hosted.
     runs-on: ${{ vars.STLC_RUNNER || 'ubuntu-latest' }}
-    if: github.repository == 'runloopai/api-client-python-staging'
+    # push: stlc builds only (Stainless-Generated-From trailer); other staging pushes
+    # wait for the schedule.
+    if: >-
+      github.repository == 'runloopai/api-client-python-staging' &&
+      (github.event_name != 'push' || contains(github.event.head_commit.message, 'Stainless-Generated-From'))
     # Optional gate: add required reviewers to this environment to approve each
     # promote. With none it only scopes secrets and adds no gate. Remove if unused.
     environment: production