CLAUDE.md: what the permanent docs hold

The new rule goes into the opening paragraph, after the sentences on docs/design.md and its chapters (new text highlighted):

Obeya: a spatial canvas for directing AI coding agents. Goal, behaviour, architecture and the decisions behind them are in docs/design.md, an index, and its chapters under docs/design/: read the index, then the chapters the change touches, and keep those current in the same change. The permanent docs (docs/*.md and the chapters under docs/design/) hold only what an experienced engineer would need to rebuild the software with the same features, however different the reimplementation would look. Work in progress is planned in plan docs under docs/plan/ (Obeya shows them as projects); when one is done, what lasts moves into the chapters under docs/design/ (or the index) and the doc goes.

Nothing else in CLAUDE.md changed.