Give write-time hygiene feedback and age out session summaries #297

See #290, section 5.

Work

• memory_store responses carry deterministic hints: an existing memory with an equal or strongly overlapping title (Similar: #48 – update instead?), and a bag that has just become stale because this child changed.
• Hints never block a store and only name memories the caller may see.
• Session summaries older than a set age drop to low weight automatically (or are listed for consolidation into a history bag); the lifecycle runs without an LLM and is idempotent.

Acceptance

• Tests for the similarity rule on normalized titles, including false-positive boundaries.
• Aging is covered by a test with a fixed clock.
• No hint reveals a private memory of another user.