Document Logging System

[devin-ai-integration]

Add documentation for the logging system including:

• Database schema
• Query examples
• Log maintenance
• Configuration options

Example additions:

## Logging
Logs are stored in SQLite at ~/.shell_sage/log_db/logs.db

Schema:
- timestamp: DateTime
- query: Text
- response: Text
- model: Text

Estimated time: 1-2 hours