If you are Claude (Claude Code, Claude Desktop, Claude in the API): read CLAUDE.md.
If you are a different agent (Cursor, Codex, Aider, GitHub Copilot, others): start with README.md. The brain itself is a folder of markdown files plus one CLAUDE.md in the working directory. Any agent that can read a project-root CLAUDE.md can run the brain. The canonical skill lives at .claude/skills/pm-brain/ for environments that support the SKILL.md format.