## What changed
- Remove the path-only `effective_skill_roots` API and its type-erasing trait in favor of `effective_plugin_skill_roots`, which preserves plugin metadata.
- Remove skill-path ancestor lookup helpers and keep namespace resolution scoped to an explicit plugin root.
- Update plugin manager and namespace tests to exercise the retained APIs directly.
GitOrigin-RevId: 507bf43bc6147bb5ab2deba4fa176bea2535159c