feat(04-01): create memory module structure and SQLite manager

- Created src/memory module with MemoryManager stub
- Created src/memory/storage subpackage
- Implemented SQLiteManager with connection management and thread safety
- Database schema supports conversations, messages, and metadata
- Includes proper indexing and error handling

Schema:
- conversations table: id, title, timestamps, metadata, session stats
- messages table: id, conversation_id, role, content, importance, embedding_ref
- Foreign key constraints and performance indexes
- Thread-local connections with WAL mode for concurrency

src/memory/storage/__init__.py

@@ -0,0 +1,11 @@
+"""
+Storage module for memory operations.
+
+Provides SQLite database management and vector storage capabilities
+for conversation persistence and semantic search.
+"""
+
+from .sqlite_manager import SQLiteManager
+# from .vector_store import VectorStore  # Will be added in Task 2
+
+__all__ = ["SQLiteManager", "VectorStore"]