github: Add native stack gateway operations

[abhinav]

GitHub's native stack and asynchronous merge APIs require REST
alongside the existing GraphQL operations. Keep both protocols behind
one Gateway so callers use repository-domain operations instead of
transport clients.

Expose only the stack mutations and asynchronous merge state needed by
the forge layer. Attach to existing pending merges by operation ID
without changing merge behavior.

Stack reconciliation needs only pull request state, head repository
identity, and native stack membership. Add a compact batch lookup that
preserves input order, leaves missing pull requests nil, and loads each
distinct native stack once instead of loading full pull request
metadata.

[abhinav]

This change is part of the following stack:

• github: Add native stack gateway operations #1396 ◀
  • forge: Add native stack updates #1397
    • forge: Add linear merge range capability #1398
      • github: Add native stack opt-out #1399
        • shamhub: Model native stack operations #1405
          • submit: Update native stacks after publishing #1400
            • merge: Use native ranges when available #1401
              • submit: Finalize partial submissions #1403
                • docs: Explain native GitHub stacks #1406

_{Change managed by git-spice.}