Preselect setup steps with pending managed config updates

[Copilot]

Rerunning suitup via curl skipped already-completed setup steps, so new suitup-managed zsh config additions—such as deferred completion loading—were not offered for incremental merge.

• Pending update detection

  • Added detection for suitup-managed files whose shipped config has safe line additions.
  • Covers .zshrc, .zshenv, core/shared zsh configs, tool modules, plugins, and aliases.

• Setup step selection

  • Completed steps remain deselected only when no managed config update is pending.
  • Steps with pending additions are preselected so reruns can show the existing diff preview and confirmation flow.

• Regression coverage

  • Added coverage for a completed zsh-config install with outdated completion.zsh, ensuring setup reruns select it for update.