The desktop routed /background through the slash worker, whose HermesCLI
prints the completion from a fire-and-forget thread after process_command
already returned - past the worker's stdout capture window - so the
result never reached the conversation that started the task (#97635).
Route the command to the gateway's prompt.background RPC (the TUI's
path) and render the background.complete event into the originating
session's transcript, mirroring the TUI's [bg <task_id>] system line.
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