Plugin packaging orchestrator (ADR 0030).
See docs/superpowers/specs/2026-05-24-plugin-system-design.md for the
authoritative design. The crate is intentionally a thin orchestrator:
it parses plugin.json, applies discovery/filter rules, namespaces
items, and hands roots + configs to the existing skill / hook / agent /
MCP / output-style loaders. It does not duplicate per-surface logic.