docs: explain when the local compare API is better than CLI output

[ykd007]

Closes #384

Added a sentence in the Compare failed and successful Browser Use runs section explaining when to prefer the local API endpoint over browsertrace compare:

Prefer the local API endpoint over the CLI output when a script, dashboard, or automation preflight check needs the first-divergence payload as JSON — for example, a CI step that reports the exact divergent action back to a tracking system, or a local debugging UI that visualises the diff without shelling out.

Also updated test_browser_use_guide_documents_compare_run_metadata_checklist in tests/test_metadata.py to assert the new sentence is present.

Verified:

uv run --python 3.11 --extra dev pytest tests/test_metadata.py::test_browser_use_guide_documents_compare_run_metadata_checklist -q
1 passed

[aaronlab]

Thanks. You claimed #384 first, and this covers the requested Browser Use guide sentence plus metadata coverage.\n\nLocal verification after merging current into the PR branch:\n- ........................................................................ [ 17%]
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401 passed in 0.30s -> 401 passed\n- -> passed

[aaronlab]

Thanks. You claimed #384 first, and this covers the requested Browser Use guide sentence plus metadata coverage.

Local verification after merging current origin/main into the PR branch:

• uv run --python 3.11 --extra dev pytest tests/test_metadata.py -q -> 401 passed
• git diff --check origin/main...HEAD -> passed