feat(restack): improve restack with better CLI flags and add tests

[jonnii]

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Stack

💡 Notice: This PR is part of a stack merge into branch stack-merge-stack-1776899834.

• PR refactor(merge): simplify multistack discovery logic #862
  • PR feat(foreach): improve foreach action and add tests #863
    • PR feat(foreach): add foreach events and dry-run support #864
      • PR feat(foreach): add stack range filtering and parallel execution #865
        • PR feat(restack): improve restack with better CLI flags and add tests #866 👈
          • PR docs(cli): update stack-fix agent template #867
            • PR docs(cli): update stack-verify agent template #868
              • PR docs(cli): update stack-absorb agent template #869
                • PR docs(cli): update stack-sync agent template #870
                  • PR docs(cli): update stack-status agent template #871
                  • PR docs(cli): update skill templates to use scoped restack commands #872
                  • PR feat(sync): add would_restack_stacks to dry-run JSON for restack planning #873
                  • PR feat(foreach): add --all-stacks and --stacks multi-stack scope flags #874
                  • PR feat(restack): add --parallel flag to run independent stacks in parallel worktrees #875
                  • PR feat(restack): annotate JSON output with stack_root per branch for multi-stack grouping #876
                  • PR docs: update restack examples to use scoped flags across user-facing docs #877
                  • PR fix: harden stack automation error handling #878
                  • PR docs: clarify would_restack_stacks must be recomputed after sync before restack #880

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