fix(tests): keep sweep import probe portable on Python 3.10

[solomonneas]

Summary

• Replace the full python -S -m brigade regression command with a standard-library package-discovery probe.
• Preserve issue tests: repos sweep health suite fails locally (6 tests, sweep_run returns 1) #542 coverage by proving the sweep subprocess receives src/ on PYTHONPATH.
• Avoid booting the full CLI without site-packages, which fails on Python 3.10 before the import-root assertion is reached.

Root cause

PR #545’s regression test disabled site-packages and then started the full Brigade CLI. Python 3.10 has no standard-library tomllib, while python -S also hides the installed tomli fallback. The test therefore failed during CLI startup instead of checking the sweep subprocess import root.

Verification

• brigade work verify run --target . --command "./scripts/verify" --capture brigade-work
• Receipt: 20260726-194952-work-verify-60c994
• Exit code: 0
• Result: 4297 passed, 3 skipped with 82.80% coverage

Follow-up to #545.

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