Add linux-lab prerequisite profile

[codeforester]

Summary

Add an explicit linux-lab prerequisite profile for Mac-hosted Ubuntu lab setup around Multipass.

Scope

• Support basectl setup/check/doctor --profile linux-lab.
• Detect Multipass presence and version/status without creating VMs.
• Keep VM creation as documented user action rather than automatic setup behavior.
• Update docs and completion/profile guidance.

Acceptance Criteria

• linux-lab is accepted by profile normalization and can compose with existing profiles.
• check/doctor report actionable Multipass findings.
• setup --profile linux-lab --dry-run shows the planned Multipass installation path without mutating the machine.
• Docs explain how linux-lab relates to Ubuntu local testing and Base's Linux support boundary.
• Focused tests and full Base validation pass.