- Rename omo-task to call-omo-agent with mandatory run_in_background parameter
- Implement background mode using BackgroundManager (fire-and-forget abort)
- Implement sync mode with existing subagent logic
- Fix background_cancel: use fire-and-forget abort to prevent parent session interruption
- Add call_omo_agent to tool disable list in explore/librarian agents
- Add call_omo_agent to tool disable list in BackgroundManager
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