* feat(memory): add structured schema and usage index
Add schema-v1 frontmatter for memory files, including stable ids, deterministic signatures, soft delete metadata, TTL handling, and migration support for legacy stores.
Track recalled memories in usage_index.json so retrieval can prioritize useful memories and auto-dream can review stale unused entries before pruning.
Keep project and ohmo memory backends aligned under the same behavior while preserving runtime compatibility for unmigrated Markdown files.
* fix(memory): make backend migration defaults explicit
Yang XiuYuan