Add a Base FAQ document

[codeforester]

Problem

As Base grows, common product and onboarding questions are starting to repeat across README sections, design discussions, and issue threads. A dedicated FAQ would give users a concise place to answer first-run and conceptual questions without bloating the main README.

Proposal

Add a top-level or docs-linked FAQ Markdown document, likely docs/faq.md, and link it from the README and docs index.

Initial topics

• What should I run on a blank macOS machine?
• When should I use bootstrap.sh, install.sh, Homebrew, or a source checkout?
• Can Homebrew-installed Base and source-cloned Base coexist?
• How do I know which basectl is active?
• Why does bootstrap print basectl update-profile instead of changing shell files automatically?
• What belongs in Base versus a project-owned installer?
• How should developers configure workspace.root?
• What is the relationship between basectl setup, basectl onboard, and basectl doctor?

Scope

• Keep the FAQ practical and user-facing.
• Avoid duplicating long design explanations already covered in focused docs.
• Cross-link deeper docs where appropriate.