load_namespaced_skills(
backend: BackendProtocol,
source_path: str,
namespace: SkillNamespace
) -> listLoad and namespace every skill found under a plugin source.
Reads each discovered skill directory's SKILL.md directly, since the SDK
loader only scans one level below a source and would not read a leaf
directory's own SKILL.md. Nested directories become :-joined namespace
segments (e.g. plugin:foo:bar:review).