feat: content deduplication on ingest via BLAKE3 hashing

[salishforge]

Summary

Hash-check memory content before hot-tier insertion to prevent storing duplicate information that inflates consolidation batches.

Design

• Compute BLAKE3 (or SHA-256) hash of content on add()
• Check hash against recent hot-tier entries for the same agent
• If duplicate detected: update metadata/timestamp of existing entry, skip insert
• Add content_hash column to hot_tier for fast lookup

Inspiration

Inspired by CCRider (MIT) BLAKE3 content-based deduplication strategy.

Closes #52