Transitioned from obsolete gateway object classes (`edge_gateway_shell_action_jobs_o`, `edge_gateway_shell_events_o`, `edge_gateway_shell_sessions_o`, `edge_gateway_update_jobs_o`) to the new agent implementation (`edge-gateway-agent/agent.php`).
This commit introduces delivery metadata tracking for gateway commands, updates, and shells. Adds preferred delivery channel handling, refined validation for edge agents, improved relay management logic, and broker presence reporting. Includes schema changes, enhanced shell handling, and test coverage.