workstation-v0 Mac-on-Linux operator runbook

[mdheller]

Closes #132.

What changed:

• Updated docs/workstation/RUNBOOK.md for the workstation-v0 Mac-on-Linux lane.
• Added install sequence, rpm-ostree reboot notes, validation commands, warning triage, and known gaps/non-goals.
• Kept the change docs-only.

Validation evidence:

• test -s docs/workstation/RUNBOOK.md
• grep -F "sourceos status" docs/workstation/RUNBOOK.md
• grep -F "sourceos doctor" docs/workstation/RUNBOOK.md
• grep -F "mac-screenshot.sh" docs/workstation/RUNBOOK.md
• grep -F "check-keyboard-policy.sh" docs/workstation/RUNBOOK.md

Known gaps:

• This is an operator runbook only.
• It does not implement doctor integration, dock validation, or shortcut map contracts.

Non-goals preserved:

• No implementation scripts changed.
• No workflow changes.
• No package manifest changes.
• No production readiness claim.