Harden contradiction, duplicate, and staleness detection

[BlueBirdBack]

Memory hygiene now exists in first form, so this issue should track the remaining work instead of the whole problem from zero.

Already exists:

• scripts/mneme_memory_drift.py
• docs/memory-drift.md
• stale / contradiction-style heuristics
• review-oriented surfacing instead of silent rewriting

Remaining work:

• stronger duplicate clustering / entity-level dedupe
• better contradiction grouping with fewer false positives
• explicit repair workflow / reviewed output format
• clearer severity / confidence scoring for hygiene findings

So this issue is no longer "add memory hygiene" in the abstract.
It is now about hardening the hygiene layer that already exists.