opencode: keep a joined call's work out of the call before it

A resume via task_id that joins a running background child is queued as the child's next run. The plugin ended every held call at the first idle, so the joined call was exported before its prompt existed and declared the full export. It now ends the oldest held call at each idle that follows a busy period (an errored run reports idle twice).

The export slice for a call now runs from its prompt to the next call's prompt, so a call exported after the child served a later one no longer takes that call's files and tokens.