Expand description
Skill gene model (M7b).
A SkillGene is a pure field-level projection of a SkillManifest. It is
not persisted — derived on demand. Two genes can be diffed and recombined
to produce a third manifest.
Scope (M7b): procedure-mode skills only. Context-only or command-only
skills are not eligible — from_manifest returns Err for them.