Remove superfluous files from repo

Benchmarks, demo artifacts, and redundant docs have no place here.
Kept: src/, tests/, pyproject.toml, README.md, SPEC.md, ARCHITECTURE.md,
SECURITY.md, LANGUAGE_SUPPORT.md, USER_GUIDE.md, TOKEN_SAVINGS.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jgravelle