Release SDK updates

[kernel-internal]

53efbf1 ci: harden promotion pull request updates
243093b ci: promote through merge-commit pull requests
191223e ci: add stlc promotion workflows

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Note

Medium Risk
Workflows can push to production/staging main and open merge PRs using a GitHub App; misconfiguration or history divergence could block sync or require manual promotion.

Overview
Adds GitHub Actions automation to keep kernel-node-sdk-staging and kernel-node-sdk aligned on a single fast-forward-friendly history.

stlc-promote.yml runs on pushes to staging main (staging repo only). It compares trees with merge-tree, skips when production already has staging’s content, otherwise force-pushes stainless/release to production and creates or updates a promote PR whose body is the commit list from production/main..staging. It enables merge-commit auto-merge (with a warning if that fails).

stlc-sync.yml adds scheduled back-sync from production main into staging when staging is behind (with an ancestor check to block divergent histories), optional prod-released repository dispatch from production on release, and on staging pushes a seal-custom-code dispatch to kernel/kernel for non-generated commits (skipping Stainless/bot commits). Dispatch steps no-op with notices when tokens are missing.

^{Reviewed by Cursor Bugbot for commit 53efbf1. Bugbot is set up for automated code reviews on this repo. Configure here.}