feat(quickjs): implement default task primitive in code interpreter for programmatic subagent calling (#592)

### Summary

This PR adds a default task primitive to the code interpreter that
allows the agent to natively write code to programmatically dispatch
subagents. The task primitive supports passing response schemas for
dynamic structured output that helps with more complex orchestration
patterns.

This PR also removes the swarm task tool in favor of the task primitive.

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Co-authored-by: Hunter Lovell <40191806+hntrl@users.noreply.github.com>