- Add AgentSession database entity with SQLite persistence
- Implement AgentSession CRUD operations in repository layer
- Update executor to use hybrid cache + database approach
- Add session restoration on startup (LoadActiveSessionsFromDB)
- Link comments to agent sessions via agent_session_id
- Fix SQLite NULL handling for nullable task fields
- Add dev database setup with seed data (make dev, make dev-db-reset)
- Update Makefile with dev targets for development mode
This enables agent sessions to persist across backend restarts,
solving the 'execution not found' error for resumed conversations.