Add `source discover` command for workstation onboarding

[rmanibus]

Summary

Implement the first rollout slice from RFC 0014 by adding a cloudstic source discover command.

Scope

• Add cloudstic source discover
• Support human-readable output and -json
• Support -portable-only
• Discover local source candidates with mount metadata and portable-drive hints
• Wire command help, completion, and user-guide docs

Motivation

RFC 0014 depends on a discovery layer before setup workstation can build an onboarding plan.

Non-goals

• Full setup workstation flow
• SSH config hints
• Google Drive Desktop hints
• Profile generation or write-path onboarding

Exit criteria

• Users can list candidate local sources for onboarding
• Output is usable by both humans and automation
• The command is documented and covered by CLI tests

Parent epic: #130