GH Stack — Native Stacked PRs

Read first: @skills/push-ci/references/authorization-contract.md before any gh stack operation that pushes — § Push safety for the credential and the unshared attestation, § Efficacy Boundary for what the per-use approval authorizes. If that Read fails, stop and report it; run no pushing operation.

Wraps the github/gh-stack extension so a branch chain becomes a real Stack on GitHub — one PR per layer, chained bases, per-layer diff view, linked merges — instead of the hand-built chained-base PR set /create-pr --stack produces on its own.

Authorization