shamhub: Model native stack operations

[abhinav]

Submit and merge integration need a deterministic forge that exposes the same
optional native-stack boundary as production providers.
Add an opt-in ShamHub repository implementing the complete WithStacks
contract while leaving the capability absent by default for existing scripts.

Plan immutable stack replacements without mutating stored state.
Execution atomically validates and replaces the touched relationships together
with their provider-facing bases, allowing submit tests to detect sequencing
regressions.

Preserve divergent relationships in storage and derive disjoint linear merge
plans from that representation.
Each executable range validates and constructs its merge or squash result
before one compare-and-swap publication, so failures cannot expose a partial
atomic merge.

Refs #1388

[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.}