Implement streaming task graph plan

[sroussey]

Implements streaming outputs for tasks, allowing partial results to be propagated and consumed by downstream tasks with configurable readiness.

This introduces a new STREAMING status for tasks and extends the TaskRunner, Dataflow, and TaskGraphRunner to manage stream lifecycle, chunk propagation, and readiness signaling. The DependencyBasedScheduler is updated to respect first-chunk or final readiness, enabling more efficient execution of task graphs by reducing idle time. New IExecuteContext helpers (pushChunk, closeStream, attachStreamController) facilitate stream production, and schema annotations (x-stream) allow tasks to declare streaming capabilities.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}

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