Improve batch typing safety

[AlexanderOnischenko]

Summary

• add a reusable sort key helper to keep batch comparison ordering type-safe
• coerce CLI path inputs to Paths before use and tighten none checks for baseline handling
• cast partial comparison data in the batch markdown test to the expected DiffReport type

Testing

• python -m pyright examples/demo_qa/batch.py tests/test_demo_qa_batch.py
• python -m pytest tests/test_demo_qa_batch.py (fails: missing pydantic-settings; install demo extras if available)

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Codex Task