Add optional confidence (float 0.0–1.0) and confidence_reason (str) fields
to both file and dir cache entries. Validation rejects out-of-range values
and wrong types. Fields are not yet required — pure schema instrumentation
for Phase 1.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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>
