docs: Explain native GitHub stacks

[abhinav]

Document automatic native-stack registration after submit, structural
reconciliation on later submissions, and atomic range merging.
Explain provider fallback and the spice.forge.github.stacks opt-out.

Describe why composition changes may dissolve a native stack before pull
request bases change, and note that queued or auto-merge-enabled pull requests
can prevent GitHub from restructuring the stack.

[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.}