Pluggable cross-session user modeling. Local SQLite (default) vs
Honcho cloud (opt-in).
Evaluation: Local A(95pts) vs Honcho B(60pts). Privacy: 20 vs 5.
RECOMMEND: Local for sovereignty.
agent/memory.py: Null/Local/Honcho backends + score/evaluate
tools/memory_backend_tool.py: store/get/query/list/delete/info/evaluate
22 tests, all passing.
Closes#322