Adds multi-expert coordination infrastructure:
- ExecutionMode enum (SEQUENTIAL, PARALLEL)
- MultiExpertResult dataclass for aggregating results
- execute_sequential(): Tasks run one after another
- execute_parallel(): Tasks run concurrently via asyncio.gather
- orchestrate_multi_expert(): Streaming think updates during multi-expert work
Supports:
- Stop-on-failure mode for sequential execution
- Partial failure handling (some succeed, some fail)
- Result aggregation with combined output formatting
- Exception handling in parallel execution
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>