Adapter from discovered plugins to SkillsMiddleware sources.
Qualify a skill name under its plugin namespace.
Nested skill directories contribute intermediate :-joined segments
between the plugin namespace and the skill name, matching the plugin skill
naming convention (e.g. plugin:sub:review).
Return skill source tuples for plugin skills.
Return plugin skill roots for skill-content containment checks.
Discover plugin skill sources, containment roots, and loaded ids.
Discover plugin skill sources and containment roots.
A discovered plugin ready to feed dcode adapters.
