Operator escape hatch for first-time validation after a Claude Code CLI
upgrade or a proxy-config change. Setting `PYTEST_K=anthropic and
basic_messaging_non_streaming` (for example) narrows the matrix run to
one cell, which is enough to confirm the worktree+uv+proxy+pytest
plumbing without burning a full $2 in provider tokens.
Cells the narrowed run doesn't touch are filled with `not_tested` by
the matrix builder, so a PYTEST_K-narrowed run is structurally safe to
publish — though in practice operators pair it with `--skip-publish`
during validation.