Add clean macOS install validation checklist

[codeforester]

Summary

Create a repeatable validation checklist for a clean supported macOS install of Base.

The product vision draft calls out clean-machine install success as an adoption signal. #356 covers user testing on an Intel Mac, but there is no general checklist for validating the supported macOS install path across brew-based and git-clone-based installs.

Scope

• Define the clean-machine preconditions.
• Cover brew-based install validation.
• Cover git-clone-based install validation.
• Cover setup, check, doctor, projects list, activation, test, demo, logs, and update-profile smoke checks where applicable.
• Record expected output or acceptance criteria without hard-coding machine-specific paths.
• Decide what can run in GitHub Actions versus what requires manual macOS user testing.

Relationship To Existing Work

• Complements User-test Base installs on an Intel Mac #356, which is specifically for Intel Mac user testing.
• Should reuse existing integration tests where possible instead of creating a second test universe.

Acceptance Criteria

• A documented checklist exists for clean supported macOS validation.
• The checklist distinguishes automated CI checks from manual user-testing steps.
• The checklist covers both Homebrew install and source checkout install flows.
• Any discovered automation gaps are split into focused follow-up issues.

Source Design Intent

Curated from the local product vision draft during #392.