Write unit tests for memory system

[xmaks82]

Description

The memory system (src/agent/memory/) currently has no unit tests. Add comprehensive tests.

Scope

• tests/test_memory_types.py — Memory dataclass, create/access/update_importance/serialization
• tests/test_memory_store.py — MemoryStore CRUD, query, search, cleanup
• tests/test_memory_consolidator.py — decay, promotion, merge, extract_semantic

Notes

• Use pytest and pytest-asyncio
• Use temporary SQLite database for store tests
• See pyproject.toml for test configuration