feat: add ATIF trajectory export

[minixalpha]

Summary

• define the Event Journal to ATIF-v1.7 projection boundary in the 0.8.x development notes
• add a headless --trajectory PATH interface that preserves text stdout and writes one owner-only, non-overwriting JSON document
• project user and model steps, tool observations, token and cache metrics, provider extensions, partial failed calls, and run terminal state
• validate representative output with the Harbor 0.21.0 trajectory validator without adding Harbor to the end-user package
• document the public CLI in both READMEs and the 0.8.x changelog

Testing

• uv run pytest -q (173 passed)
• cd benchmarks/harbor; uv run pytest -q (6 passed)
• uv run python -m compileall -q src tests
• Harbor 0.21.0 validator passed generated successful and failed trajectories

Note

The Chinese 0.8.x development note is the edited source in this PR. Its generated English counterpart remains unchanged per maintainer direction and should be refreshed before landing.