Clearer error when a plugin's name is invalid

A plugin is identified as plugin@marketplace: the plugin's name, then the marketplace (the catalogue it comes from). If you install a plugin, or one of the plugins it depends on, and its id is malformed, Claude Code now shows an error that says:

• which half of plugin@marketplace is invalid
• which characters are allowed
• that the marketplace's maintainer can rename it

For the npm marketplace, the error says the first half is not a package name that marketplace accepts.