feat: add workstation setup dry-run preview

[rmanibus]

Summary

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

What changed

• add engine-backed workstation onboarding plan builder
• expose the plan through Client.PlanWorkstationSetup
• add setup workstation CLI with dry-run preview and JSON output
• suggest OS-aware workstation folders and include portable drive candidates
• update usage, completion, README, and user guide
• add unit tests for engine, client, and CLI behavior

Notes

• write mode is intentionally not implemented yet; the command currently requires -dry-run
• formal sub-issue attachment under RFC 0014: Epic / Tracking issue for workstation onboarding and profile scaffolding #130 is blocked by token permissions, but this PR addresses Add setup workstation --dry-run onboarding preview #194 and continues the RFC 0014: Epic / Tracking issue for workstation onboarding and profile scaffolding #130 onboarding RFC work

Closes #194
Refs #130

[codecov]

Codecov Report

❌ Patch coverage is 81.07345% with 67 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/engine/workstation_setup.go	76.79%	33 Missing and 9 partials ⚠️
cmd/cloudstic/cmd_setup.go	75.26%	16 Missing and 7 partials ⚠️
cmd/cloudstic/main.go	0.00%	2 Missing ⚠️

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