feat(v2): add safe AI fallback and cache/reanalysis decision

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

logic/cache.js

@@ -0,0 +1,8 @@
+function needsReanalysis({ memory, currentHash, now }) {
+  if (!memory) return { needsAi: true, reason: 'no_cache' };
+  if (memory.last_context_hash !== currentHash) return { needsAi: true, reason: 'context_changed' };
+  if (memory.next_review_at != null && now >= memory.next_review_at) return { needsAi: true, reason: 'review_due' };
+  return { needsAi: false, reason: 'cache_valid' };
+}
+
+module.exports = { needsReanalysis };