Wire --held-out into the skill-optimizer SKILL.md, guide flags/safety tables, and
the tutorial's bundled-skill step: mutating a bundled skill in place now requires
--allow-mutate-bundled AND --held-out (>=5 benchmark-disjoint tasks) or it
hard-refuses. Add the --held-out flag row + F11 held-out gate to the guide; update
the receipt contract to the honest baseline/test-score fields.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
