mmr v1.6.1 — fix doc-conformance auth check

Fixed

• doc-conformance channel auth check always failed. The probe ran scaffold --version, which exits 1 on scaffold installs without a --version flag — and 1 is in the channel's failure_exit_codes, so the && chain reported auth failure even when scaffold and Claude were installed and working. Switched the probe to the scaffold version subcommand, which works on every scaffold release. (The channel is disabled by default, so impact was limited to configs that enable doc-conformance.)