compare: include run metadata compatibility hints

[kaizeenn]

Summary

• add run comparison metadata extraction for Browser Use run pairs
• include browser_use_version, browsertrace_version, model_provider, model, and prompt_template_version when present
• return compare_metadata.left, compare_metadata.right, and field-level compare_metadata.differences in compare output
• keep extraction best-effort and backwards-compatible for traces that do not store metadata

Why

Issue #369 requests a first useful compare experience that includes enough run/config metadata to decide whether two runs are fairly comparable before reading action-level diffs.

Testing

• python3 -m py_compile browsertrace/compare.py tests/test_cli.py
• manual runtime check with temporary trace DB to verify metadata extraction + differences payload

Closes #369