docs: add a Prerequisites section (install/setup requirements)

[blafourcade]

Document what a user needs before installing (supported tools/versions, runtime, accounts). Today install steps assume prerequisites implicitly.

Surfaced during roadmap review 2026-06-29.

[blafourcade]

Done. README.md now opens with a ## ✅ Prerequisites section (Node, pnpm, tool CLIs). Shipped by #407 (docs: rework the documentation — install-first, concise), merged into next on 2026-07-07.