Nest handoff history by default

[jhills20]

Summary

• add a nest_handoff_history flag to RunConfig and call a new helper that condenses the prior transcript into a developer-role summary when handing off
• update the default handoff path, docs, and helper library so that the developer summary is produced automatically unless a custom filter overrides it
• expand the handoff-focused tests to cover the new behavior (including helper unit tests) and update existing expectations

Testing

• uv run pytest tests/test_extension_filters.py
• uv run pytest tests/test_agent_runner.py -k handoff
• uv run pytest tests/test_agent_runner_streamed.py -k handoff

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https://chatgpt.com/codex/tasks/task_i_68ff73bda0f4832496f3d1fa9103905f

[rm-openai]

All of kaz's feedback seems right. In addition, I'd say two major things:

1. This setting should be on the Handoff object
2. Since you're updating the default, this is a breaking change, so please update the changelog.

[jhills20]

moved to Handoff, added handoff history .py file for non extension parts of handoff filter, added changelog