fix(agent): persist retention metadata

[knqiufan]

Summary

• Fix AgentMemory persistence so metadata.retention_score and metadata.importance_level are populated from canonical metadata.intelligence fields before Memory.add(..., infer=False).
• Add shared metadata helpers and make CLI cleanup read metadata.intelligence.current_retention when the legacy top-level score is missing.
• Add regression tests for multi-agent, multi-user, CLI cleanup, and preserving multi-user privacy_level during metadata merge.

Test plan

• python -m pytest tests/unit/agent/test_multi_agent_crud.py tests/unit/agent/test_multi_user_crud.py tests/unit/test_cli_manage_retention.py -q
• git diff --check HEAD~1 HEAD

Closes #1143

[wayyoungboy]

LGTM