Release SDK updates

[kernel-internal]

f026272 ci: harden promotion pull request updates
b94cd58 ci: promote through merge-commit pull requests
9e39767 ci: add stlc promotion workflows

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Note

Medium Risk
Automation writes to the production SDK repo (GitHub App token, force-push to stainless/release, auto-merge); misconfiguration or ancestry drift could block sync or require manual promotion.

Overview
Adds GitHub Actions to move the Go SDK between staging (kernel-go-sdk-staging) and production (kernel-go-sdk) while keeping a single ancestry line (merge commits only, no squash on promote).

stlc-promote.yml runs on staging main pushes: skips when production’s tree already matches staging, otherwise force-pushes staging main to production’s stainless/release, creates or updates a “Release SDK updates” PR into production main (body = commit list), and enables auto-merge with a merge commit (warns if that fails).

stlc-sync.yml adds the reverse path and hooks: back-sync (cron, manual, or prod-released dispatch) fast-forwards staging main from production when staging is behind, with an ancestor check before push; notify-back-sync from production on release optionally pings staging; seal-dispatch on staging pushes (non–Stainless-generated commits) optionally notifies the config repo via seal-custom-code.

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