docs: add Browser Use compare API curl snippet

[xclear-cast]

Summary

• add the local compare web API curl snippet to the Browser Use guide
• note that tools can call the endpoint while the local UI is running to get the same first-divergence payload
• extend the Browser Use guide metadata test to keep the snippet and wording present

Closes #380

Testing

• PYTHONUTF8=1 uv run --python 3.11 --extra dev pytest tests/test_metadata.py -q (401 passed, run from an ASCII temp worktree to avoid Windows CP949 path/site initialization issues)
• PYTHONUTF8=1 uv run --extra dev pytest tests/test_metadata.py -q -k browser_use_guide_documents_compare_run_metadata_checklist (1 passed)
• git diff --check

[aaronlab]

Thanks. This is scoped exactly to the Browser Use guide and metadata coverage, and it keeps the compare API path next to the CLI compare commands where readers will look for it.