Keeps repo root lean: CLAUDE.md is the only doc at root. All
reference/architecture material lives under docs/.
Also updates all cross-references in CLAUDE.md, docs/README.md,
and the FastAPI override note in code_guidelines.md so links stay
valid after the move.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>