The legacy _isSkillsRuntime gate was a hardcoded isCodex || isCopilot || ...
roster; zcode was absent so the layout-driven skills-install path was skipped
(commands + agents installed, but no skills). Adding || isZcode would violate
AC#3 (no runtime === 'zcode' branches), so the gate is now derived from the
descriptor: a runtime takes the skills path iff its scoped artifactLayout
declares a skills kind. Behavior-preserving for all 15 existing runtimes
(verified by their install contracts); includes zcode (and any future skills
runtime) with zero per-runtime branches. opencode/kilo keep their specialized
combined path.
Regenerate the golden-install-parity fixtures: the non-zcode runtimes shift only
the shared model-catalog.json hash (zcode was added to the catalog); zcode now
includes its skills tree.
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