docs: implementation plan for README quickstart rewrite

Plan landed in commits a57e8bb, b6c51e3, 4aa1cae: rewrite README
as a quickstart-first guide, correct --local + cloud subcommand
references, collapse cloud billing verbs into a details block.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: markovejnovic