config.py centralizes phase-2 path/env reads (no hard-coded data paths,
section 15). prompt.py loads the EXTERNAL, versioned prompt pack (section
14): pack version + per-prompt content hash feed prompt_version for the
response-cache composite key (D6); missing/invalid packs raise a clear
PromptPackError, never a silent fallback. Minimal fixture pack under
tests/fixtures/prompt_pack/ pins CI to a local, versioned pack.
