Add installer smoke tests and rollback guidance

[Yuning598]

Goal

Make the one-command macOS setup safer to change and easier to recover.

Acceptance criteria

• Add a non-destructive or dry-run validation path.
• Smoke-test generated Ghostty, Yazi, and Lazygit configuration.
• Document files that are linked, replaced, or backed up.
• Document uninstall or rollback steps.

Portfolio

Area, priority, and scheduling are maintained in the Workspace Portfolio project.