The unused re-gate planner `restack` leaves: the queue's stacked-prefix search already re-gates a red batch #6651

Replaces #6534. src/checks/restack.ts is imported only by its own test; judged in the merge queue already searches a red batch by stacked prefixes, and test/merge-queue-search.test.ts ("an 8-pull batch with one bad pull: the 7 good land … 2 gate rounds") proves what #6534 asked for.

Done:

• [ ] src/checks/restack.ts and test/checks-restack.test.ts are deleted, and nothing under src/, test/, scripts/ or docs/ names restack or afterVerdicts afterwards
• [ ] No other file changes; every merge-queue test passes unchanged

Files: src/checks/restack.ts, test/checks-restack.test.ts

Risks: none beyond a stale doc reference; grep for the names before pushing.

Before pushing: bun x tsc --noEmit and bun test test/merge-queue-search.test.ts.