github: Add native stack opt-out

[abhinav]

GitHub native stack APIs may be unavailable or undesirable for a
repository. Add the configuration-only spice.forge.github.stacks mode,
enabled by default, and make off report the optional forge operations
as unsupported before any gateway access.

Submit can then skip stack updates, while merge uses its existing ordinary
pull request fallback. No command-line flag or environment variable is
advertised for this policy.

[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.}