Moves investigation ID persistence and _CacheManager class from ai.py
into a dedicated cache module. No behavior changes.
Moved: _load_investigations, _save_investigations, _get_investigation_id,
_CacheManager (all methods), _sha256_path, CACHE_ROOT, INVESTIGATIONS_PATH.
Also added a local _now_iso() in cache.py to avoid a circular import
(ai.py imports from cache.py).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
