Add linux-lab prerequisite profile

[codeforester]

Summary

• Add the linux-lab prerequisite profile for host-side Multipass setup/checks.
• Document the macOS Multipass lab flow and the boundary that Base does not create or mutate VM instances during setup.
• Update profile help/completions, finding ID docs, AI context, and tests.
• Replace stray shell run calls in setup install paths with direct command execution.

Fixes #1396

Validation

• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_dev/tests/test_engine.py -q
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats --print-output-on-failure cli/bash/commands/basectl/tests/setup.bats
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats --print-output-on-failure cli/bash/commands/basectl/tests/check.bats cli/bash/commands/basectl/tests/doctor.bats cli/bash/commands/basectl/tests/onboard.bats cli/bash/commands/basectl/tests/completions.bats cli/bash/commands/basectl/tests/ci.bats
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest tests/test_doctor_findings_docs.py -q
• BASE_HOME=$PWD BASE_PLATFORM=macos PATH=/usr/bin:/bin PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m base_dev setup --profile linux-lab --dry-run
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash env -u BASE_HOME ./bin/base-test
• git diff --check
• zsh -n lib/shell/completions/basectl_completion.zsh