Plugin root errors

Name	Requirement
plugin_root_missing	The selected path must exist and be a directory containing a plugin.
archive_plugin_files_missing	A skills-only ZIP must contain a supported plugin manifest and at least one valid skill.
plugin_root_ambiguous	ZIP must contain exactly one plugin root, either at the archive root or in one top-level directory.
plugin_root_has_siblings	A ZIP with a top-level plugin directory must not contain sibling files.