Add reviewed PR stacks to Contribute

[hamzamerzic]

Summary

• group complete plan.stack chains into one ordered review while keeping every incremental body and diff independently inspectable
• add an explicit second confirmation that publishes only the enumerated ready layers and preserves partial retry state
• visualize chained PR routes, forks, working files, and source differences in the repository map
• document the immutable stack preparation and approval contract

This is layer 2 and builds directly on the repository-map layer below it. It uses the companion platform stacked-submission service.

Testing

• npm test (22 passed)