refactor: share CLI checkout navigation helpers

[jonnii]

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Stack

💡 Notice: This PR is part of a stack merge into branch stack-merge-stack-1777947741.

• PR docs: clarify scoped restack agent wording #886
  • PR refactor: share PR navigation renderer #887
    • PR refactor: share branch comparison revision resolution #888
      • PR refactor: centralize stack graph access #889
        • PR refactor: share CLI branch argument resolution #890
          • PR refactor: share CLI checkout navigation helpers #891 👈
            • PR refactor: share best-effort action snapshots #892

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