The Desktop Plugins hub had no uninstall door: the plugins.manage RPC
only offered list/toggle/install/update, so a plugin installed from the
hub could only be removed from the CLI or the dashboard. Add a remove
action that reuses hermes_cli.plugins_cmd.dashboard_remove_user_plugin
(the same core as hermes plugins remove: deletes the tree and its
install metadata atomically, refuses bundled plugins and paths outside
the plugins dir), and regenerate the shared wire contract.
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