Support plugin roots in the host skill loader

[copyberry]

Support plugin roots in the host skill loader

What changed

• Carry plugin identity, namespace, root, and discovery mode through host skill loading.
• Apply the owning plugin namespace and IDs to loaded skill metadata.
• Respect direct-child discovery for Agent Plugins and reject skills that resolve outside the plugin root or are not regular files.
• Preserve recursive discovery and symlink behavior for legacy plugin roots, and allow the full 64-character namespace plus 64-character skill name.

Testing

Add host-loader coverage for plugin metadata, namespace ownership, direct-child filtering, path containment, recursive symlinks, shared assets, and maximum-length qualified names.