Release SDK updates

[kernel-internal]

53eb143 ci: harden promotion pull request updates
710780c ci: promote through merge-commit pull requests
bf366f0 ci: add stlc promotion workflows

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Note

Low Risk
New CI-only workflows with guarded git operations and optional secrets; no application runtime or customer-facing code changes.

Overview
Adds GitHub Actions automation for the Stainless staging (kernel-python-sdk-staging) and production (kernel-python-sdk) Python SDK repos so they stay on a shared, merge-commit ancestry.

stlc-promote.yml runs on staging main pushes: compares trees with git merge-tree, and when production is behind, force-pushes staging to stainless/release on production, then creates or updates a promote PR and enables merge-commit auto-merge (explicitly not squash/rebase). Uses a GitHub App token for cross-repo write/PR permissions.

stlc-sync.yml adds complementary jobs: back-sync (scheduled/dispatch) fast-forwards staging from production/main when content differs but ancestry allows; notify-back-sync on production release optionally fires prod-released to staging; seal-dispatch on staging pushes (non-generated commits) optionally notifies the kernel config repo via seal-custom-code. Optional dispatch secrets degrade gracefully with notices.

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