Add setup workstation --dry-run onboarding preview

[rmanibus]

Summary

Add the next RFC 0014 workstation onboarding slice: cloudstic setup workstation --dry-run.

Scope

• add setup workstation command scaffolding
• implement a dry-run-only onboarding plan builder
• suggest OS-aware workstation folders
• include portable-drive candidates from source discover
• expose the plan via the public Client
• render a review-first human-readable preview and JSON output
• update usage, completion, and docs

Notes

This issue intentionally stops short of the interactive/write path. The command should remain preview-only for now and return an error without -dry-run.

Parent RFC/epic: #130