fix(desktop): identify automatic merge results as notices #3793

Automatic merge completion for PR #3792 appeared as a user-authored chat bubble containing raw JSON. The current mergeTracked callback in apps/desktop/src/state/host.ts sends an unmarked prompt through deliver, while regular pull updates use the persistent pull notice marker.

Acceptance criteria: merge completion and failure wake the thread as automatic notices, never user bubbles; a readable summary is displayed; the complete result remains available to the agent and attribution survives history reload.