submit: Update native stacks after publishing

[abhinav]

Submission can create or update change requests without refreshing a
forge's native stack metadata. After successful publication, reload the
branch graph and send each affected tracked tree to repositories that
implement WithStacks.

Use BranchGraph.Bottom to find each affected tree and collect its
published changes. Treat an unsupported capability as a no-op, and warn
for other update failures without changing the successful submission
result.

Model the capability in ShamHub behind an opt-in configuration so
end-to-end tests can observe complete, order-independent native stack
trees without changing existing script behavior.

[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.}