docs: document retention tiers for agent memory

Author: Aetheris-Labs

docs/retention-policy.md

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+# Retention Policy
+
+Engram mixes hot retrieval, deduplication, and pruning. The storage policy should stay easy to reason about so memory quality does not drift over time.
+
+## Hot tier
+
+- Keep recent facts that are still likely to affect the next agent decision.
+- Prefer concise entries with clear timestamps and sources.
+- Rewrite duplicates into one stronger entry instead of stacking near-copies.
+
+## Warm tier
+
+- Move items here when they still matter for context but no longer need first-page retrieval.
+- Keep relationship pointers so the retrieval layer can promote them again if they become relevant.
+
+## Cold or expired
+
+- Demote notes that have not been referenced and no longer change behavior.
+- Drop low-signal fragments that fail both retrieval usefulness and audit value.
+
+## Operator rule
+
+A memory item should survive only if it changes what the agent would do next or explains why a past decision happened.