docs: document faircode benchmark --root and --no-plots (#1040)

[minutechreview]

Summary

faircode benchmark --root PATH and --no-plots only appeared in --help. This adds both to the README benchmark examples, with a short explanation under them:

• --root PATH is the directory searched for */audit.yaml (one level down, default the current directory). It is ignored when manifest paths are passed, which matches run_benchmark in faircode/benchmark.py.
• --no-plots skips figures/*.png, so matplotlib is not needed. The three CSV files are still written.

I also added the check the issue suggests, in tests/test_cli.py: every long option of every subcommand has to appear in the README or faircode/SPEC.md. It fails on the old README (benchmark --root, benchmark --no-plots) and passes now.

I put the text in the README only. SPEC.md has no benchmark command section today, and the new test accepts either file. Say so if you want a SPEC paragraph as well.

Type

• Audit
• Explainer
• Bug fix
• Other (docs and a test)

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Linked issue

Closes #1040

Checks: pytest tests --ignore=tests/browser gives 817 passed, 21 skipped. ruff check faircode tests scripts, scripts/check_em_dash.py and scripts/check_broken_links.py pass. I did not run faircode benchmark itself (scikit-learn is not installed here); the descriptions come from the argument parser and run_benchmark, discover_manifests and write_report in the code.

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@yakew7 @Shreyash0712 @ahmdkaml - new PR touching a path you own, please review.