Remote-control bridge gains an 'onBackgroundTasks' hook

The remote-control bridge now wires an onBackgroundTasks callback into session setup, alongside existing hooks like onDialogKindsDeclared, so background task state can be reported to remote control/bridge clients. A new background_tasks control_request subtype lets an external caller background a running local shell tool by tool_use_id, and remote clients can query or clear background task state via the task registry.