build: keep repository clutter out of source archive

[abrichr]

What

• reduce the source archive from about 11 MB to 952 KB
• keep CI, tests, scripts, the Chrome extension, the obsolete Whisper plan, and the large demo GIF repository-only
• preserve current documentation and required package metadata
• extend the distribution validator so these paths cannot silently return

Why

The public sdist should be a professional, reproducible source artifact rather than a mirror of repository-only development material. This lowers download and storage cost without deleting any repository source or changing runtime behavior.

Validation

• built the exact wheel and sdist from this head
• both archives pass scripts/verify_distribution.py
• wheel rebuilt through the cleaned sdist matches the direct wheel byte-for-byte
• focused repository/runtime contract tests: 7 passed
• Ruff, actionlint, and git diff checks pass
• no package publication requested