Luca

For [Mercury](https://github.com/lucabro81/mercury-fw) plugins that work through a command-line tool: it turns the command line the model writes into an argv array of its own (never a shell), runs it only when it matches the plugin's allowlist, and stages

What a [Mercury](https://github.com/lucabro81/mercury-fw) plugin author imports: the contract a tool plugin implements (`Plugin`, `PLUGIN_API_VERSION` and the types around them) and the one a channel implements (`ChannelPlugin`, `CHANNEL_API_VERSION`), in